You need something with an aluminium frame & alloy wheels, not steel. Avoid mountain bikes or anything with suspension. I would go for a hybrid bike (or city bike) - which is much easier to pedal than a mountain bike - the tires are thinner, but not as thin as a road (racer style bike) so a bit more robust.
Maybe something like this, this has a steel frame, but it is a lightweight one - aluminium frames tend to be more expensive, and I am not sure what your budget is.
http://www.discountbicycles.co.uk/biz/product. php?xProd=220&xSec=4