hi Chase. Sub 5:00 mile at age 50 is a great goal. 20 seconds is a big chunk, but you finished really strongly, and you said it was your first mile race, so it seems realistic to expect to improve.
Take this with a grain of salt, as I've never coached anyone over the age of 13, and I've run the mile maybe a dozen times, but for what it's worth I got my "masters PR" of 4:47.44 [only race all year our club times to the hundredths of a second!] at midsummer night's mile '06 after doing the following workouts in the preceding 3 to 4 weeks. My goal had been 4:50 so I figured 72 secs./lap and a 2-second cushion to cover those last 9 meters. Each workout was planned as 3200 meters at goal pace.
1. 16 X 200 (200 jog) in 36. Actual average 34.6
2. 8 X 400 (200 jog) in 72. Actual average 71.99 [no sweat!]
3. 200-400-600-800 (400 jog)-600-400-200 [200 jog except where noted]
couldn't hold desired pace after first 600. 36-71-1:46-2:31-1:52-74-37
4. 1200 time trial (3:34.7), full recovery, then 5 X 400 (200 jog) averaged 72.8
5. 1600 time trial (5:01, gagging on last lap), full recovery, then 4 X 400 (200 jog) average 72.6
In previous years I'd usually just done a 1600 time trial to decide what heat to enter [varied, but often I ended up running the race 8 to 10 sec faster] and 8 or 10 X 400 as main workout.
In the '06 race itself I went in the fastest heat and was dead last in 70 at 400, up to 9th of 11 at 800 in 2:21, last two laps 73 each [last one being a bit long of course].
Having Jim H. as a pacer is a great idea. My best miles have included a pack to chase and someone I was trying to catch in last lap [Alisa Harvey in the '06 race]. Really concentrate on not letting the pace slip in laps 2 and 3, and let it rip.
I hope others with more track and/or coaching experience will chime in. Good luck in your quest.