With the OP only claiming to have been riding for maybe a little over two years, this might just be a matter of expecting too much too soon.
More miles more often while riding on their own or with one or two other friends will help them get to where they wish to be more than likely. As will picking some hills and flats or false-flats to go all out on during those rides done outside the big group.
No HR or PM necessary. Though it is nice to be able to document through data the improvement.