Running

Alternatively you can take a change of clothes and an extra lunch in on a Monday (on your bike or whatever) and run in on a Tuesday. And if it's safe to leave your bike, you can cycle in, run home one day then run in the morning and cycle home the next, so that you're not doing back-to-back runs on the same day.

This is what I do, works a treat.

If the distance is too far then not much you can do about that I guess...

Commute in part of the way on public transport then finish off by running (i.e. get off a few stations/stops early). If no public transport then cycle a hack bike part of the way, lock it up and then run the remaining distance.

If in London then run as far as you want then use a Boris Bike for the remaining distance (I did this when I was building up to my full 7 mile run commute).

Or, get into work early and then go for a run (or go for a run at lunch).