To expand on what HUB said for Tmcalist...
I like fields that have markers, which tends to be soccer fields. These are often either 300' or 330', and usually there are fences just past the lines or other landmarks for better judging further distances. I'll pace out approx how far I think these landmarks are, and afterwards Google Earth/map it to see how far it says. After throwing at multiple places, if you're getting the same #'s within 10' or so, you can be pretty sure you're throwing that far. Now I can throw at a new field and throw either a mid or something like a Firebird and have a pretty good idea if it's a 300' or 330' field and go from there. Once you know your general disc distances, which are probably within 15' of the same distance on any normal shot with that disc, you'll know how far you throw and be really good at judging what to throw and when.
I like throwing all my discs in practices, and never just focusing on high speed stuff only. If I do that and go back to putters I'll be torquing them. Also it's a good idea to throw every disc you have on slight hyzer, flat, and slight anhyzer releases. You'll learn way more about their flight paths and see which discs respond to angles the best (does it want to go straight and power through a hyzer release? Does it want to go left no matter what? Does an anny release give you a smooth left to right or does it give you a flex shot to increase distance and still go straight-ish?).
As far as your distance...throwing an anny flex type shot will possibly give you a bit more distance, but I consider this bonus distance. It's not increasing your power, it's learning a new (and more difficult in some ways) line to throw. If you're throwing 350-375' with drivers, and probably up to 350' with fairways, you're at a very common plateau. The way to throw farther is to increase arm speed from here...and that is done by getting into better athletic positions to transfer and accept your weight which will rocket your arm out there. Anyways, that's what I commonly think holds people back when they are at that distance, and it is what it was for me as well. Having to learn to have better balance and weight transfer. Watch Sidewinder22's stuff, you're at that point now.