First of all, 2:40 male marathoner is in no way a sub-elite. I'm a minute faster than than that and am still just a club runner, barely competitive in local races. There are guys with full time day jobs who can just outclass me on the race day leaving no chances, even if they bonk I won't catch up those 10-15 minutes. It can even be argued that 2:40 is not sub-elite for females ffs.
I have a home treadmill and use it a lot during winter for easy and moderate running. Doing 'workouts' on it is not sustainable. There are two reasons for that. First, the lowest I can keep my home gym while keeping the rest of the house is about 55 degrees, and even at that with a large fan on I would overheat and be drenched in sweat. Second, the higher end speeds are not sustainable. Marathon pace - both by feel and by footpod measurements - can be 10.2 mph setting one day, 10.6 later that same day when the motor heats up and 10.4 the other day; it's still somewhat usable, but everything faster is even more all over the place with top setting of 12 mph sometimes giving actual pace around 5:30/mile, so lower than my 5K pace. My treadmill is a 3hp model, retailing for around $1000.
These two factors combined, I would always at least try to do a 'workout' outside or on indoor track and only use treadmill as last resort. Long runs are also undesirable as sweating increases exponentially and becomes unbearable at 80-90 min range. But unless you call all runs 'workouts', I won't hesitate to jump on it for an easy one. Of course, running outside is more fun and generally feels easier, but you have to get through that part of getting out of the house into the terrible dark and cold before it becomes fun and comfortable. Also, safer and no snow on treadmill.