Media playback is unsupported on your device Media caption Dan Smith trains for the London Marathon in Zambia

On Friday, Bastille revealed the title track of their third album: A foreboding, state-of-the-world pop sermon called Doom Days.

"We wanted this song to be really direct in addressing our modern anxieties," says frontman Dan Smith. "Phone addition, porn addiction, rolling news, Brexit and Trump.

"Turns out there was a lot to talk about. I wrote 50 verses, but somehow I managed to cut it down."

The song is the first taster of Bastille's new album, which Dan previously described as an "apocalyptic party record".

A follow-up to 2016's Wild World, it's due out later this year, and the band will celebrate with their first ever performance on Glastonbury's Pyramid Stage.

Before all that, however, Dan has the small matter of a 26.2 mile race to complete.

The singer is taking on this weekend's London Marathon, raising money for Breast Cancer Now, a charity that helped his mum when she was diagnosed with the disease.

He's had to fit the training around the band's world tour, which meant venturing out in 29 cities around the world, from Melbourne to Munich, as well as a pit stop in Zambia.

"That was truly amazing," he says, "apart from the mild threat of black mambas and snakes and aggressive ostriches."

Ahead of the race, the star talked BBC News through his preparations, the making of Bastille's third album, and how their biggest single was originally intended for Justin Bieber.

What's the furthest you'd run before entering the marathon?

I did a half marathon years ago, and I remember crossing the finish line and thinking: "I am never, ever going to do anything like this again in my life."

So what changed your mind?

My mum suffered from breast cancer and, luckily, having gone through treatment is in remission.

As a family, we feel insanely lucky that she's ok, and I had the strong feeling that I wanted what happened to my mum, in terms of her treatment, to be the same for every family that comes into contact with breast cancer, or any type of cancer.

Do you remember the moment you signed up?

I was watching the marathon last year and the buzz of it is very inspiring. So I said in passing to my mum, "Ah, maybe I should do the marathon," not expecting to be taken seriously.

But, prior to being diagnosed, she was chair of a charity called Breast Cancer Campaign, and helped it merge into Breast Cancer Now. So she came back very quickly to say they were really excited about me running, and I was like, "Oh no - I've got to do it now."

Image caption "My bandmates are looking at me in wonderment and horror"

How did you find training amidst all your shows and the travel and the jet-lag?

I get quite nervous on stage, and the way I get around that is by moving around a lot - running and jumping and my awful attempts at dancing. So that was something to think about; and I worked with a trainer, Elkie, who's been amazing with helping me fit these long runs around the tour.

What was the worst experience you had?

One particular incident in Spain, I got back from a run and bumped into our crew at the hotel. They were kind of laughing at me, but I wasn't sure why until I got in the lift and saw in the mirror that I'd killed about 50 flies with my face throughout the run!

I looked like the windshield of a lorry on the motorway - although that implies I'm a much faster runner than I am.

How did you find training in Zambia?

Zambia is at a higher altitude than Table Mountain, so that was quite tricky. But I felt very lucky to look out over a completely different environment and see giraffes and zebras wandering around.

Another benefit to all this exercise: You can eat whatever you want.

Yes! My bandmates are looking at me in wonderment and horror as I have three portions of everything whilst my body just shrinks and shrinks.

I imagine as soon as the marathon finishes and I stop running that I'll balloon to quadruple the size.

Have you got a target time for Sunday?

I'd love to finish in under four hours; but ultimately the most important thing is to raise money.

On tour, we've been collecting through donation buckets at our merchandise stand and, early on, I said I would match all the donations we got - which I'm very happy to do, but it's been interesting watching the total go up!

Running aside, how's the album coming along?

I've been specifically told I'm not allowed to say anything about it, but it feels like a really important album for us. It has a lot to say and it's quite conceptual, so we'll see what people think!

It's following a one-off single, Happier, that just clocked up six months in the US top 10.

I know. Isn't that insane?

Did you expect it to do so well?

Well, I actually wrote Happier for Justin Bieber and everybody in our team fell in love with it, and persuaded us to keep it.

But because it's a collaboration [with Marshmello], we haven't done a huge amount of promo, so the success is something that's happened at one remove, like an out-of-body experience. So when Happier was at its peak, we'd taken two weeks off and I was backpacking in India, which is pretty surreal.

Image copyright Getty Images Image caption Bastille have moved up Glastonbury's bill from the John Peel stage in 2013, to the Other Stage in 2016, before arriving on the main Pyramid stage this summer

It's going to go down a storm on the Pyramid Stage. How are you feeling about Glastonbury?

I literally can't believe it. Glastonbury is my favourite festival in the world. It's something me and my mates have done every year since we were at uni and I love it so much. It's so inclusive, and it's so culturally diverse.

Are you planning anything special?

We're trying to put together a show that's quite different and interesting and fun for people to watch. If we're playing that stage, we're going to do it properly.

But I'm also really excited about it being over so I can watch Lauryn Hill, who's on after us, and then Stormzy - which is going to be something else.

Given Lauryn Hill's reputation for time-keeping, you might want to prepare an extra hour or two of material...

Haha - imagine!

It's so surreal for me. When I was growing up, The Miseducation of Lauryn Hill was my sister's favourite album, and the first record I ever owned was The Score by The Fugees, Those albums changed music, I think, and changed popular culture.

So to play before her, and to play on the Pyramid stage is like a double hit of mind-blowing prospects.

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