This is just my personal opinion: If it was me I would use a track bike with a racing gear ratio for sprint training and I would have another fixed gear bike set up with a much lower gear ratio for doing recovery rides and road rides...Sprinting is an explosive activity so you want to focus on developing power, explosive strength and speed...IOW focus on practising sprinting rather then wasting time and energy on doing long distance endurance rides A little bit of endurance work is ok but I would keep my road rides no longer then 2 hours. Too much endurance training in not good for sprinting because it will compromise and weaken your fast twitch muscles and explosive strength both of which are essential for track sprinting. If possible, commuting on a fixed gear is a great way to get some base miles.