Hill running in London

Where are good safe (well lit and public in the evenings) hill spots to do training in London? I work in the city and live in west london so can be relatively flexible ... just not too north or south ideally.

Also if it has to be done in the gym, what's a good session to do?

Running my first marathon next year. 4.5 hours realistic. (4 hours the dream). 

Problem is you don't really want a hill with other people getting in the way. Are you looking for grass or paths? I always trained at Parliament Hill sometimes on paths and sometimes on the grass slopes. Not sure that would meet your criteria though, although if you were in a club session that would give you some safety. You could get there on the Northern line city branch after work, not too long a jog from Kentish Town tube.