What is the best training plan to be able to improve on group rides in the 45-50 mile range? I already have the stamina for the distances, but don't have the speed for the groups that I'm trying to hang with.

My main question has to do with frequency of rides and rest. For example, if I ride 150 miles per week, I can do that in 3, 4 or 5 days, like this -

5 days:
Four 25’s, one 50
4 days:
Two 25’s, two 50’s
3 days:
Three 50’s

What do you think?

And I fairly randomly picked the 150 miles/week. Should I go more? Or is that too much?