I’m not really sure how this is different than your original post, unless you’re looking for specifics. My first option would be to get one of those water running belts (some clubs have them) and do water running. A long workout will test your mental toughness. You might not want to spend 3 hours on an elliptical trainer, but 3 hours outside on a bike would go by fairly quickly. You could always mix things up – 1 hour on the elliptical trainer, an hour on the bike, then an hour on the stairmaster. Get creative.
Also, I don’t know exactly what your injury is, but I think with a little down time, you’d be able to bounce right back. It seems we tend to hear lots about knees, IT bands, achilles, etc. Ankle injuries seem to be more related to lateral sports. So my guess would be that 2 weeks from now you won’t even remember you were injured.
At this point in time, I think you could afford to take a week off if you think it’ll help you heal. That’d still give you 2+ weeks to get your running legs back. You could even squeeze in a medium-long run like 13-17 (if you are healed) miles during those last 2 weeks.
What you don’t want to do is take 2 days off, think you’re healed, then reaggravate your injury by running on it. This will only force you to take more time off and possibly miss Boston. Nip this in the bud now and you can still toe the line in Hopkinton.
5 weeks ago I had to take 8 days off because of my achilles. I kept telling myself to cross-train, but I never did. I started to question whether or not I should even run Boston. 1.5 weeks after that time off I ran 20 miles and all those negative thoughts have disappeared.