Craig, if you are buying a brand that you are familiar with how the sizing is, you might try Kelly's Running Warehouse on the Web. They have some great prices and are located in San Diego. Shoes usually get here in a couple of days.
Intervals on a track can be boring, but it is easy to measure the distance and timing. I am the rare guy who likes interval training on a track, but my personality gravitates to accounting and computer programing, I like precise organization in my life.
You might want to try Fartlek training. You pick out a landmark and gradually pick up speed till you are almost at racing pace (not quite) and hold that pace for about 30 seconds. Then walk or jog about 1/2 the distance you just covered. Keep repeating it. Most distance runners use this technique. You see more track interval training for middle distance runners (1,500 - 5,000 meters). But if you like the track, and one is convenient to you, there is nothing wrong with that kind of training. With your first 50 miler, I wouldn't worry about much speed work.
After track season my son takes a week off in June and then runs all summer averaging around 100 mpw. He won't do any speedwork until he has run a couple of races. Then he will back off the mileage a little bit, and increase the speedwork so he peaks at conference and the NCAA's. He is really a middle distance runner (up to 5K), but made it to the NCAA Div I finals in cross country the last 4 years. College cross is 8K races until the regional qualifying meets and finals, where they run 10K. It has been run watching him the past 9 years, and we talk training a lot. Methods haven't changed much since I was seriously racing 40 years ago.
A little bragging here... in high school was 2nd in the state in cross and the mile (1600m) his senior year. Best times were
In college he has done
1500 3:46 (many times, he is stuck here ~ 4:04 mile)
He hasn't done many 5000m races, and is going to concentrate on it this year.