I find that the biggest obstacles for achieving this - for me - are location, occupation, sweating, and migraines.
I run very hot (menopausal) so even 20 mins worthwhile exercise will make sweat pour from my scalp, leaving my hair damp and frizzy for at least an hour which then needs fixing. I may also get a face flush so I'll look like a tomato to boot, and my face leaks sweat so I can't put make up on for an hour. About 50% of the time exercise triggers a migraine. So doing anything before work means allowing 2 hours, plus migraine risk.
I also don't work in one place - I go to different locations every day. One day it could be 30 miles away, the next it could be 100 miles, so my leaving times vary from 5am to 8.30am. I don't get any lunch break on site and parking is stressful enough when you've never visited the client before, without researching where to park 1.5 miles away and allowing an extra 30 mins (and anyway, see sweaty tomato/migraine issue above).
After work is feasible, although if I'm driving 75 miles, doing a full day on site with clients with no break then driving 75 miles home and getting in at 7pm, I really don't fancy walking anywhere!
I need to do something though.