i am pleased to see more cyclists, i myself love cycling and used to compete alot until i got BIG, i also worked in various cycle shops in my teens. I thought id join the natter.
The good thing about a bike is that it is usually rather painless way of doing cardio, i mean running always seems to hurt me (plus i only have feet with one speed).
To thos worrying about the bum ache, my advice is that yr butt does get used to it by your 2nd or 3rd consecutive ride, some helpful aparatus would be. As suggested a gel saddle cover, or a gel saddle although the gel saddles are very restrictive to the motion of yr legs. What i find helpful is to buy the padded insert for under yr pants, these help cushion you.
For people worring about a bike which will hold them, i £99 dual suspension will cope with alot of punishment, just ensure you tighten the rear coil as this can cause a very tough ride for the newbie as alot of yr exertion is absorbed in the suspension.
Cycling is great at burning calories and i recommend it to anyone.