It can be done, maybe....
First you need to look at a route, is it possible without going on Motorways, a quick look at Google maps, makes it look possible, but Google maps is not known for it's accuracy outside the United States, it's better then Apple Maps, but that's not saying much.
Second, you need to look at your volunteers bicycles, anyone with a proper road bike (even an old one), has possibilities, anyone with a BSO, take a pass.
Third, you need to look at your volunteers physical condition, if they are in good shape, then they are possible, the 5 stone overweight, 50+ guy, uh, no.
Fourth, you need to look at your volunteers families, anyone with a child under the age of three, forget it.
Fifth, you need to look at your volunteers, job. If it's a guy who works regular hours, okay, if he works 60+ hours a week, he will not be able to make the time commitment.
Now take the guys you have left, drive the route, look for the steepest hill along the way, have them ride back about 10 miles, then U turn and ride back, so the last thing they get to is that hill. This is the last test, if anyone makes it, they have a chance.
Now they need a bike computer, they need to start at 10 miles per ride, riding 4 times a week, with one of the rides on the weekend. The weekend ride, gets 5 miles added on, every week, from now until August. They will then be able to handle 180 miles, easy.