I agree with Jenn. I've been doing some really tough/intense 90 minute to 120 minute intervals and be completely wasted afterwards. But by morning I feel fully recovered. I alternate an intense interval workout with an easy workout b/w. I'm older than you, so you should be recovering more quickly!

Have you tried any recovery drinks or compression tights to help your legs recover? I don't use compression tights myself, but I do make sure to have good nutrition after a workout and to work the knots out of my legs with a rolling pin. You could also try eating foods to encourage muscle repair and reduce inflammation: Cruciferous veggies, cucumbers, etc.