Desk pilot,
We're probably not dissimilar in height and weight, but unfortunately mine is due to too much beer and not enough exercise.
However, whilst completing my ATPL last year I elected to buy a bike and cycle each day from my accomodation to the airfield, about 2 miles and fairly level ground. In only 3 months I lost almost 7kgs without altering my diet or social life (read beer!). As someone who doesn't much enjoy running, I found cycling an ideal exercise, and thoroughly enjoyable, especially on a really good summer afternoon.
I suspect the main problem you may have though could relate to C of G limits in light aircraft if your FTO expects you to fly with a training partner as well as your FI. I found during my training that I couldn't fly certain aircraft with 3 on board and full fuel due to weight or C of G limits, but fortunately there were always alternative aircraft which were well within limits.
Whatever happens though, best of luck with it all.