My girlfriend is trying to run sub 3 marathon this October at the Hudson Mohawk Marathon. It's flat and fast - but legal! I think she has a lot of potential as she has run multiple marathons "for fun" in 3:10-3:13 (off about 50-60mpw). She has been running since she was 18 - 24yrs old now. I think off effort alone should could improve 5-6mins since she typically runs them hard but loses focus because she loves engaging with the crowds during the race - giving them high 5's etc.
That said - I've increased her mileage a little (about 60-70mpw) and have been giving her track workouts (800s, 400s, 200s) since she has very little speed. For example - when her 5k pr was 19:15 - her marathon pr was 3:15 which I think is way better.
Besides the track stuff - I'm going to be incorporating longer tempos into her training as well. She will eventually be running 15-18 mile "tempos" to prepare for the marathon. Her long runs get up to 23-24 pretty easily as she is an endurance machine.
Could you guys give me a few workouts to give her that if she completed would be good indicators she could run sub 3. I'm thinking of stuff like 3x2mile repeats, 2x3mile repeats, 3x5k repeats etc. She's extremely tough - not much can really break her.