Considering your level of fitness across multiple sports plus the history of having centuries under your belt, I would think you have time to train enough to do this. Perhaps your hardest hurdle will be mental versus physical, but if you want to do it, it will be within your reach. You are an accomplished rider and that speaks for itself.

About the couple time riding issue, well I don't have much to comment on here except to say, if it's possible, I'd try to find some riding buddies besides your DH right now. Who knows, this might prove to be very enjoyable for you, more than you might think.

Best of luck whichever choice you make.