Getting out to the trails took about 30 seconds, I was 4 minutes ride from Hebden Bridge, I worked between 60 and 90 hours a week so I didn't really take advantage.
I then moved to Leeds city centre, just off the top of Briggate, I could ride my MTB to Meanwood and be off-road in 15 minutes.
Now I'm in London, it's a faff, I don't drive so need to get the train out, usually to Peaslake and that area but in the last five years my MTB rides have tumbled and I've started riding the Touring and road bikes more. It would only really take an hour and a bit to ride out of London from mine, but it wouldn't be very good riding once I was there and I'd have to ride back obviously.
So, long story short: some cities are very open to green spaces and the MTBing is easy, London, IMHO, not so much.