I've just paced a football field enough times that I've practiced what a 3' stride feels like -- so when I want an approximation, I count the steps to the disc, multiply by 3, and there is my answer.
I think that practicing a 3' stride that you can use for distance measuring purposes is easier than Olorins method of trying to do the math to figure out how long your natural stride is -- for me, my "natural stride" depends on my mood, the shoes I'm wearing, the terrain, what I had for breakfast that morning, etc... my practiced stride I've found to be pretty consistent. +/- ~3% margin of error. So it's good enough to be helpful in most situations.
If you've got a football field available, use that. You get solid measurements marked for you out to 360', as long as your throwing from goalpost to goalpost. This also gives you a target to aim for that helps you make sure your are throwing straight instead of wildly to either side of the line you are aiming for.
Very few disc golfers throw too much over 360' so for throws out the back of the endzone, you can just pace off the number of steps past the endzone. So if you are 22 paces out the back of the endzone, thats 22x3=66+360=426. And when the first 360' is measured for you, theinconsistency of your steps for the remaining distance will be really insignificant compared to the total distance being measured, so I'd guess that you'll get measurements that are accurate to within 2% that way -- plenty good enough for seeing your throwing game develop.
Your real-world distance is not the distance on the one disc that got a random fluke wind or crazy flip and went an extra 40' past everything else you threw. I'd take 5 discs and throw them all. Eliminate the longest and farthest throws and average the other 3 distances -- see what that works out to and that's probably a good number to use when people ask how far you can throw. And there really aren't many golfers who can throw for real much past a football field with any consistency -- probably not more than a few thousand at this point.
This is exactly what I do.