Washington Secondary is ok. One problem is that the Warwick sewage treatment plant is very active and leaves a residual smell on the path from May through August. I will be happy when all the connections are made that will extend through Coventry. Right now it is only an 10 mile path which is a little short if you are trying to really exercise.
I wanted to try and ride around the reservoir this year but I waited too long. By the time I had a weekend day free, it was foliage season and frankly I don't trust the leaf gazing drivers. However, this is an awesome ride. It is a lot more hilly than the bike paths so you have to be in some shape.
I ride the East Bay path once a week during the summer if at all possible. This is a 36 mile ride for me 3 miles to and from the path and then I do the whole path and back to my house. sometimes I extend the ride by coming up through east Providence and doing the ten mile river greenway which makes it 40 miles or so, and sometimes I do a 50 mile ride which involves a loop in Seekonk. the East bay is the best for exercise because other than the 2 miles between 114 crossings you don't have to stop. Washington Secondary is fine but it's urbanity means it crosses a ton of roads, and there are a lot of backyards where folks treat the path as part of their backyard. The last time I rode it in the summer I had to stop and walk my bike around a family who had set their grill up with a picnic table on the path behind their house.
One thing I do for exercise is lap Blackstone Blvd. This is a 3.2 mile loop and I usually do between 18-30 miles. There is a slight grade so it is good exercise especially if you force yourself to stay in one gear. the only problem is the people parking at the Lippitt Park end who have no idea that anyone else is around. the great thing about this is that when you are done you can stop at Maximillian's and put all the calories you burned right back on.
for major roads in Providence, biking sucks because a lot of utility work happens on the sides of the road. I don't own a road bike anymore and won't invest in one because the stiff suspension would kill me. I frequently ride up Hope St. from Pawtucket and I'm surprised my teeth don't rattle right out of my head.
I don't ride downtown because of my aforementioned (in the thread frankie linked) problems with finding a place to lock the bike.
The Blackstone path should be open this month to bike traffic where the bridge is being worked on. I like the blackstone path also because it is easy to exercise, and also if you are lucky you can be on the path racing a train which is fun. I can't wait for it to be connected to Worcester, which would be a nice 50ish mile round trip ride without having to go on surface roads.