Approaching the big 40!

maybe you can do repeat 20m runs something like 2-3x10x20m. that would give you a decent 200m worth of sprinting per set. You should feel pretty lactic doing that. then maybe add in some power work maybe put a sand bag or wieght plate in a felt bag tie to a belt and you have a good indoor sled that wont scratch up the floor… as for tempo you can get creative…