Yep, what sulis said, you can certainly do the 64 miler if you start training now.

The key is to gradually increase the distance you ride over the next 6 weeks.

The general rule of thumb is to increase by about 10% or so each week. The other rule of thumb I've seen (where did they come up with that term anyhow?) is that if you can comfortably do about 75% of your target distance before the ride, you'll be fine. So you'll want to be able to do about 50 miles at one time by the end of May.

You say you're up to about 24 miles right now, so here's a sample plan (assuming you do your long ride on Saturday). It's not exactly a 10% increase per week but close enough.

4/21: 28 miles
4/28: 32 miles
5/05: 36 miles
5/12: 40 miles
5/19: 45 miles
5/26: 50 miles
6/2: LRRH

Also, during the rest of the week you should try to fit in two or three 5 mile rides (gradually increasing one or more of those to about 10 miles) so you don't do just one single long ride one day a week.

Hope this helps! - Jo.