well i started being a ciclist 2 years ago and it took me a lot of training to start noticing changes, the main changes were on my legs. anyhow ive always said its not the time u run, u can run up to 3 hours at a turtle speed and u wont do anything with that. its written on my training calendar, when i first started, i used to run 2.64 miles in 23 minutes. right now i can do a lot more than that in the same time cuz im more prepared than when i started, what i mean is, maybe u get to ur job in 45 mins, maybe i get there in 30, maybe some1 else gets there in 15 mins, its about the miles ur running and the speed ur running them, it is not the same to do 30 crunchs now, take a 30 minutes break and do 30 more, than to do 60 crunches straight, you excersise more if u do them all at once. same goes with this, its not the same running 3 miles at 30 mins than running em at 15 mins, the excersise is better if u do it faster cuz ur excersising ur legs more.
my advise is to keep time of it. before you start to run, put a timer to run with you, and stop it when u finish, write down ur time every time, when u get to ur job, write ur time, when u get to ur home write it again. dont try to beat ur record, if u go to work every day on bike u will break the time on ur own and u will see day by day that u r doing it faster and faster.
i hope i was helpful. best of luck
P.S- jefflove is right with what he sayd