Yeah, unfortunately pain is par for the course when re-starting exercise after a long period of none whatsoever. At least for people who are, as you say, no longer 25.
(I'm assuming the pain is muscular, if not, you'll need to see a doctor to get it properly diagnosed.) The previous poster made the assumption that it's calf pain -- I'm guessing the same myself, since I've felt it. There's only two things you can really do for it. First, build up very slowly. If your legs will only allow five minutes (or ten, or 30), then only do five minutes (or ten, or thirty). As frustrating as it is, in retrospect you'll find it's really not very long until you can tolerate more. And avoid hills/avoid running (if you were considering it), those are just even harder on the calves.
Second, you may be able to do some exercises to strengthen those muscles so that you build up your abilities more quickly. If you google for calf strengthening exercises or exercises aimed at preventing shin splints, that's what you're looking for.
Good luck! The best thing about being so out of shape (I say, having been there myself) is that you see improvements very fast! You can do it.
Height 5'8 1/2"
GW: Originally 150. Maintaining at about 146 since June 2013.
5K 4/21/11: 31:55
|40 Maintenance Weeks