half hour training?

i work stupid hours and work away alot. i can take my bike, but especially during winter i might not finish until 8pm and it's dark.
what would be the best type of training to fit in in around half an hour to boost fitness? would it be some sprint type interval training? or just a 90% effort half hour blast? or something different?
cheers guys
what would be the best type of training to fit in in around half an hour to boost fitness? would it be some sprint type interval training? or just a 90% effort half hour blast? or something different?
cheers guys

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I'd do something different each time I went out.
The dark also isn't an obstacle with some decent lights, in fact it's a damn good laugh.
i know that, i have lights. but the problem is when i'm away i usually only have about half an hour!!!