If you're coding, writing, doing research based tasks or basically anything where you're at a computer or in a book, the difference going for a walk makes is incredible. It's taken me about 4 years to accept that even the hardest problems sometimes fall into place during a shower, while walking to get a coffee or just through going somewhere physically different and talking through the problem to yourself - no pen, paper or screens allowed.
I don't get too much out of breaks every 45 minutes, and when things are going well am happy to work 2,3 even 10 hours straight, but if I've been stuck on something for more than 20 minutes 9 times out of 10 if I leave and come back I can solve it pretty much instantly. It's like a reset button to my mental state, and I'm always pleasantly surprised with the result.