I guess I should have expected everyone here to just say, "go for it".
Wondering if anyone has any anecdotes of people that have almost exclusively cross trained because of injury before a marathon and still performed well.
@SteveC702 You know anyone?
Part of my apprehension is not just because of the injury but at this point I'm not sure what's realistic either.
It depends on how long you were cross-training beforehand and if you are actually sufficiently healed. (and later part only you really know, and you don't really know until you are 20 miles or so into the race). I have done this twice, and it didn't work out particularly well either time.
The first time was my first marathon where I was forced to cross train from 28 days out up until the week before with a foot injury. Then 8 days before the race I did a short run (probably similar to your 20 minute run) and felt good, so I went out and did 2x3 miles at 6:00-6:05 pace the next day. I also did a 5-mile tempo run about 4 days out from the race and a 6-miler the day before. (I was young and apparently an even bigger idiot than I am now). I started the race with a goal of running under 3 hours, but by 5-6 miles my 6:45-6:50s turned into 6:20s and I rolled with it up until about halfway when my foot (same injury that forced me into cross training a few weeks earlier) started hurting. I still averaged 6:40/mi through 20 miles before the wheels completely came off and I jogged/walked a 54ish last 10K for a 3:07.
The second time I tried something like this was Boston in 2013. The deck was stacked even more heavily against me because I still wasn't healed. I popped about 2-3 ibuprofen pills an hour before the race and off I went. I was okay up until about halfway (1:17) and then everything started tightening up due to changing my form to compensate for my injury. I walked/ran the last 10K again.
The only case really famous case I know where somebody pulled this off was Carloes Lopes who won the 1984 LA Olympic Marathon after supposedly not taking a step for at least a week leading up to his race because he got hit by a car 7-10 days out from the race. Nobody really knows how serious his injury was though, and his training was pretty much done anyways and he just missed out on the last week or so of his taper. I actually had a similar case to his where I mysteriously jammed my hip/glutes 5 days before Rock n Roll Arizona in 2012 and couldn't even jog for 3-5 minute without excruciating pain until about 24 hours before the race (and I could still feel it jogging around a 9-minute mile pace the day before). I started the race with the intention of dropping out if things ever got bad and ended up finishing with a 5-minute PR in 2:37. However, I didn't have to cross-train much before this instance either since my mysterious injury came on the week of the race and seemed to have gone away just in time.
My only advice if you want to do this is:
1. You are probably still in decent shape if you have cross-trained seriously, but you absolutely want to start at the slowest end of your goal pace range.
2. If your injury has healed sufficiently, your legs are likely going to feel better and more fresh than you usually do for a race - even one when you have fully tapered - due to the fact that you haven't run very much. Don't let this fool you into picking up the pace too early, wait until 20 miles or so at the earliest.
3. Have backup (drop out) plans ready - even if it's just carrying a credit card of some $$$. If you injury starts rearing its head and all your time goals have gone out the window, do not finish. I ended up not being able to run pain free for about 3-4 months after both cases I cited above, definitely not worth it.