I was in pretty damn good shape when I was at the construction company busting ass 10-18hrs a day in 100F+ weather in CA, AR, and any other place hot they always sent me. You sweat off any excess fat and kept pretty stong because it was very pysical work.
Now they sent me to the engraving shop (close to being an office job, sitting if front of a computer drawing stuff and then hitting the go button and engraving it) and after about 9 months there I gained about 20lbs and lost some strength. About a month a go I started kyaking more often and bought some dumb bells and work with them for about 15mins every night with some sit ups. In about a month I lost the 20lbs and gaining my pysical strength again. Back to being 6'2", 195lbs, toothpick lookling basterd.
Once you start getting back in shape you start getting more energy/will to do more things it seems....