I have a Gregory rucksack which seems to do the trick fairly well, but the best rucksack will depend on your body shape so it's worth trying on a few at a running shop and jumping up and down a bit. It was fairly expensive, about £80 from memory. A couple things to bear in mind are that running rucksacks aren't terribly breathable so they will make you quite hot so I'd tend to wear a layer less than I would otherwise, and secondly even a few lbs extra weight seems to make a big difference to how easy my run feels. If you're planning on changing at the gym, it might be worth investigating how much it is to hire a locker there as my current commuting strategy is to leave clothes, shampoo, shoes etc at my locker at the gym the day before, then run in with a belt thing which carries keys, phone and oyster card. Alternatively, running home and leaving stuff in work tends to avoid the logistical issues of getting work clothes to the right place, but running into work is a great way to start the day and is actually quicker then getting the tube.