Run for the hills.
That Johnson has chose to set out his availability now rather than wait until nearer the election and parachute into a safe seat courtesy of a late retiree isn't exactly a ringing endorsement of the skills of his former adviser Lynton Crosby and his "Kill Mill" strategy. Johnson clearly believes regardless of his transparent suggestions to the contrary that Cameron's position should the Tories fail to win is likely to hang by a thread; indeed, you can only really see Cameron remaining leader in the event of a Labour minority government. A Lib-Lab coalition or outright Labour victory, distant as the latter might seem at the moment, and he would surely be forced to resign.
Johnson is far too canny though to just wait it out, hence the report he commissioned which finds although staying in the EU would be in our best interests, it wouldn't be a disaster to leave either. Despite Cameron's placating of the right of the party by getting rid of the few remaining holdouts against leaving the European Convention on Human Rights, Johnson is positioning himself as the man to really get us out of the EU, whereas Cameron won't even contemplate his renegotiation strategy failing. The one problem with this is Johnson is just as socially liberal as Cameron and Osborne (he could hardly not be) something that doesn't sit entirely comfortably with the headbangers who believe the only thing holding the Tories back is a "real" right-wing manifesto and leader.
Whether we'll see Cameron and Johnson on the campaign trail together isn't clear, although you have to suspect we won't be seeing a Tory version of Cameron buying his rival an ice cream. Those with links to the "players" are making out this was all agreed in advance and how delighted Dave and pals are by Boris deciding to battle on two fronts. Underneath George Osborne must be seething, his own designs on the leadership having clearly been thrown into the utmost jeopardy. As for where this leaves Londoners, destined to have a part-time mayor for over a year in spite of Johnson's past promise to not do precisely this, they've never really been at the forefront of his considerations anyway.
The other question is whether the country as a whole could really stomach Johnson as leader of the Tories, let alone prime minister. You have to go back almost half a century to find someone else with such a how, shall we say, distinctive past in a similar position. We prefer our leaders if not boring then certainly staid; our most recent flirtation with someone even closely comparable didn't end well for us, although it certainly has for him. The "Boris ler-gend" cult can only go so far. Should the Tories not win next year however, making it 23 years since they last got a majority, longer than the 19 Labour spent in the wilderness, they might just be moved to decide Boris couldn't possibly make things any worse.