15 mins is heaps, at my gym all the really fit guys do it in 1 mins bursts! I have read that if you excerise the upper limbs, followed by lower, followed by upper etc it burns more calories than long sessions on each- all that blood rushing around?- and my circiut classes are all upper/lower/upper/lower based on this. Perhaps you could do five mins bike, one min arms, repeat endlessly?
Have a good day James!