I did it for 2 years - 2 hours door to door 5 days a week! Tell you what, it was nowhere near as bad as people had me believe.
Left home at 6.15am, walked into office at 8.15. Lunch at desk, out the door at 5pm ON THE DOT AND NO LATER (as a pp said, you need a very thick skin for this and be confident that your career progress won't be affected, otherwise you'll just fret). Back at home station just after 6.30 and in the door by 7pm.
99% of the time, no problem at all. You can watch stuff on your phone, listen to music or audio books, I even crocheted and set up a fb page "commuter of the day" to document the odd and amusing things that I saw. V entertaining.
The other 1% is a bit shit, yes. Sometimes the trains go tits up and you have to get creative and get on a bus (spell of 2 weeks in 2 years after a line collapse) or cope with immense and sometimes hysterical crying, sweltering crowds (twice in 2 years). Sometimes there are delays (I recall maybe 4 or 5 occasions, but rarely held up for long).
Your train operator makes a big difference - I was South West Trains and they are not bad at all, but Southern have big problems.
The worst part for me was the mad dash at 5pm to get to the train early enough to get a seat - standing is miserable. People are a bit crabby and miserable at rush hour, get a thick skin and walk FAST! I got plantar fasciitis in week 1 and it was chronic for the entire 2 years.
Worse than all of that? Needing a poo on the train. I have IBS and even had a bowel infection whilst working in London, but developed incredible bowel control...apart from the time I had to make a dash for it at a random stop en route. 
If you're generally fit, can keep a positive outlook when all around you are miserable fuckers in a rush, don't mind being pushed around a bit, hot or squashed sometimes, can handle sudden inexplicable changes to your routine, and don't mind very short evenings at home...I say do it, but don't plan to continue for more than a couple of years! It's doable but nice when it stops. 