Hey! When my ferritin was 11 had other important blood values on the lowest end of normal (but not technically anemic... Even though most were less than .5 from below normal, lol) I couldn't even walk across my very small campus without taking a break because I would see stars.
I took iron, ferrous sulfate tablets, on an empty stomach 2-3x a day with orange juice and avoided calcium within several hours of taking iron like the plague. I cut down most distance runs in both pace and duration and focused on interval work because my endurance was already trashed from such slow running by the time I found out about my low ferritin. Added to distance runs veryyyyyyy slowly (like 5 minutes more every other week) and in about 6 weeks, felt substantially better. Ferritin was in the low 30s.
TL;DR don't expect to keep 80mpw for the short term and be okay with that. Lots of iron supplements with Vitamin C. Focus on working on what you can, and be patient when you feel like crap. It'll come back if you do the work.