I'm training for a marathon in October and I'm trying to plan out my long runs a bit. I've heard of a lot of people "hitting a wall" after mile 20 in a marathon, and for a lot of them their longest long run during training was around 20 miles, which makes sense. I was thinking of building up to a slightly longer distance, like I would want to do 22-24 once before the actual race to kind of avoid/delay hitting the wall. My plan was gradually building up to it by adding 2-3 miles per long run until I get to whatever the longest distance I choose is, and then dropping back down the next long run to something like 16-20 miles and holding it around that until I need to taper. So my questions are basically:
1) is this a horrible idea? 2) if this isn't a horrible idea, what would be a good upper limit to set on long run distances? 3) for a farther long run, I was planning to go roughly at my goal pace and up to ~15s/mile slower. Would that work, or should I slow down more than that? 4) if I were to do a farther long run like this, roughly how long before the race should I try to do it?
This is purely hypothetical at this point, so I don't have specific details on how this is going to fit into my training plan overall yet, and I'm open to pretty much any suggestions/advice because I quite honestly have no clue what I'm doing. For a bit of background: I'm 19F and have run competitively and pretty consistently for about 8 years, the longest run I've done so far was 15 miles in one go (at roughly 1 min/mile faster than my marathon goal pace, but I was admittedly in better shape then than I am now) and 18 miles total in one day (did a 5K park run with a few friends, then ~1 hour later one friend and I did a 15 mile long run), so I'm not completely new to longer runs but I'm also not experienced with things in the ballpark of 20+ miles.
Edit: sorry a lot more people answered than I expected, I really appreciate everyone taking the time to respond, I'm trying to reply to people but I've also got an exam to study for lol
By - Peppermint_Sonata