If you want to train distance runners get your Ex.Sci or Kines. Degree, and your NSCA-CSCS then go work as a strength and conditioning coach.
NSCS-CSCS and the ACSM-HFS are the gold standard. Both require a bachelors. NASM is the lead in the next tier. Lots of good certs (more $ for NASM). If no degree, get the NASM and NASM-PES. ACE is garbage. AFAA is garbage.
In a personal training setting you will likely never work with serious runners.
I worked in personal training and corporate wellness. If you want benefits like vacation days, health insurance, or 401k work in corporate wellness.
It's a fun job, but you will work mostly from 5-7am and then from 4-8 pm.
Unless you own your own studio you will not make more than 30-50k and that's a stretch. Well, unless you live in a high cost of living area, which 100k = 50k anywhere else.
90% of your clients will be well to do 30-50 year olds, mostly women.
Alan