How hard is a 40 mile trek in 24 hours?

I'm kind of athletic, do over 200 push-ups and sit-ups a day and run at least 2 miles also, and I was thinking about joining the SAS. I'm still young so I can still prepare for it. A 40 mile run in under 24 hours was a part of the test, and I was wondering how hard that actually is. Can someone describe or tell me please?