LONDON — A pale but vigorous-sounding Prime Minister Boris Johnson returned to work on Monday, declaring that the coronavirus that nearly killed him was like an “unexpected and invisible mugger” the British people had begun to wrestle to the floor but had not yet fully disabled.

For all his determined brio, Mr. Johnson’s message to his lockdown-weary nation was somber, underscoring the hard choices that Britain faces as the economy languishes and the death toll from the virus soars above 21,000.

Mr. Johnson, who was discharged from the hospital only two weeks ago, signaled that the government would keep some social-distancing measures in place for the foreseeable future. To lift them too soon, he warned, would mean “not only a new wave of death and disease but also an economic disaster.”

“This is the moment of opportunity,” Mr. Johnson said at Downing Street, before a full day of meetings. “This is the moment when we can press home our advantage. It is also the moment of maximum risk.”