amazingtrade said:
PS I am sure this title originaly had a D in it.
Afraid not... Was wondering when you'd notice that.
I'm a similar age to you and I recently hit the same thing, where it was obvious I needed to do something to combat sideways growth. I blame canteen lunches.
So, according to anquet mapping, today's ride was 24.5 miles, taking in 2000ft of climbing (a lap of the hereford beacon), in hot humid conditions (akin to riding in a sauna). I'm on holiday this week... the ride was to counteract the barbecues held on saturday, sunday, monday and yesterday, all washed down with much beer (and the odd glass of wine).
Riding a bike will give general condition and good tone in legs (and upper body if you're doing proper mountain biking, if you're just pottering along roads and canal towpaths you won't get this), but it won't flatten your stomach. For that you need to do situps and leg lifts.
Walking rather than taking the bus is good... In southampton I found it was quicker to walk the 2.2 miles from my digs to town than to catch the bus. There was also no bus to uni, which was 1.4 miles (and 140ft climb), so I'd typically walk 4-6 miles a day while I was a student.
Making a good start now is important. If you can keep in shape now, it will be easier to do so later.