Having checked out this thread using the search feature, I did the Bear Mountain ride today for the first time (solo), start/finish ~central park, just went up 9W (except near Nyack where I went along Piermont). Ok, so I only got halfway up the mountain -- the last bit of road was closed and covered in gravel and it was getting late (I started at 11AM), so I wasn't too upset about turning around. In terms of distance, it was my longest ride by far, but it's also flatter than northern California so I thought I'd be fine; timewise, it was only ~30 minutes longer than I'd ridden before, but I was working a bit harder here since I was doing all the pulling. It took about 6 hours 10 minutes, including breaks. The roads were clear (except for that one road up to the top, maybe there's a different way?) and the weather was great, I think it was at least a couple degrees above freezing the entire ride, clear skies, almost no wind. I saw several groups heading south from Nyack on my way out, but somehow I only saw one road bike going the same direction as me the entire ride.
The hardest part of the ride for me were the hills on the way back -- I was tired! But the bottom half of Bear Mountain wasn't bad at all, although I'm sure it would be harder to do the whole thing -- but there were several climbs on the route that were significantly steeper. I think the hardest hill was the one just south of Bear Mountain, but so much of the ride is more or less flat that it would definitely be fun in a group.