How would you adjust your marathon goal in this scenario (missed 3-4 key training weeks)?

I'm not qualified to answer, but 2 weeks off + 2 weeks cross training doesn't mean you've lost that much fitness compared to before your injury, especially with a few more chances to get in a workout before you taper.

All I can do is speak from personal experience. I tend to lose and gain fitness pretty quickly. 2 weeks of no training and 2 weeks of sketchy training would majorly impact my confidence and fitness. In your shoes I would probably look to keep volume up next week as well and maybe do a Saturday long run (maybe 17-18) with some decent M pace work. Maybe like 4x5k w/ 1k float or something. I would re-evaluate off of the this weekends workout and that one. I personally would probably target 2:42:30 or so. I’m sure others will disagree but this is what I would do.

My original take would be to find a different marathon 4-8 weeks after the original date but I get it with Boston. 2 weeks off is tough but not a killer. 20-30mph + the elliptical work also isn't a killer. I don't think you've lost so much fitness that you'd have to bump from 2:35-2:40 to 2:50. 2:50 is any given weekend kind of shape for someone running 2:35-2:40.

2:35-2:40 is kind of a big range. I've been in 2:37 shape and have known 2:35 is too fast for me. I think you have to be honest with what you would have targeted without the time off. Think about your BQ, PB, & fitness leading in. Maybe you've run 2:35 or faster, I have no clue. If you think closer to 2:40 would have been a good day, maybe sub-2:45 is the call now. Boston is a tough course, even if some people don't think it is. Even if you were in 2:40 shape, I might recommend someone aiming for that to go out closer to 2:45 pace through 5k. You also could get bad weather and have to make an adjustment.

My take would be to go out at 2:45 pace through 5k, maybe even 10k, if the weather looks decent. Nobody has ever finished Boston and said, "I wish I would've bombed the opening 5k in Hopkinton/Ashland." If the legs aren't quite there, the more conservative start will help. If you feel good, the easier opening miles should help you run well. 6:15ish pace should feel easy in the first 10k of that course. Your legs will thank you in Newton. ~1:00 positive split is actually ideal in Boston. 1:21:30-1:22:00 opening half (with the slower early miles), 1:22:00-1:23:00 or faster back half. Something like that to give sub-2:45 a go. If it feels super easy you can still toy with sub-2:40 with a 1:21 opening front half. Think there's just a lot more risk going out hotter.

Thanks all for the thoughtful responses. I think I'll just see how this weekend's workout goes and make a call. I think the wildcard for me here is how much fitness I was able to maintain with the elliptical, so if anything this will be an interesting test.

NERunner, I did think about just doing a different marathon, but I'm already planning on doing another later in the year, and this will be a first Boston for me, so figured I'd just aim to get to the start line in the best shape possible, even if not PR shape, and just reset for the next marathon.