My husband weighed 173 pounds when we met in college and he just jumped my scale and he weighes ---- 173 pounds, 25 years later. He is around 6' 2" and so is long and lanky. He really can't put on weight if he tries and he is very, very quiet about that because he understands it's a sore point.
About 6 years ago, he started having real trouble with his knees (they are hinged a little funky) and had laproscopic surgery on them both. The doctor recommended bike riding as a really good way to build up the muscles around his knees to help support the mildly funky joints. Very shortly after that, we moved to a very bike-friendly community and he now commutes to work by bike every day that it is possible to do so. He also hikes a lot for fun. He has changed his eating some to incorporate more fresh fruits and veggies. In short, he is probably in better health today at 46 than he was when I met him at 21!
Now, if we could only ease him back off the Type-A, workaholic part of his personality......