I read that the Yasso test is a good indicator of marathon time.
15 min warmup
do 10 x 800m reps separated by a slow jog to rest and recuperate
if 800m time < 3:30, rest time between reps is 2:00 to 2:30
if 800m time > 3:30, rest time between reps is 2:30 to 3:00
throw out the fastest and slowest times and average the 8 remaining times
if average time is 3min 35 sec, estimated marathon time is 3h 35min