You should ask first timers.
Also - are you able to get stats on each page? If so, which are not being hit? Do you want them to be used more? Or is everyone "telling" something by their choices?
Can you get some coworkers/friends/family who haven't been to the site to give you good info?
I think that keeping people here seems to be a non-issue. Of course I don't know what you really want - number wise.
In terms of the look-feel, I think the phpBB is almost so ubiquitous that it is user friendly because we all know it well enough. The non-phpbb part could seem more closely related to the phpbb in style (just color and stuff).
Real hardcore opinions:
Take the New! off of the logging link. Call it "Chart Your Progress" or something.
Move the Community link to a separate place not on the main menu and give it a bit more of a descriptive title and prominent style - newcomers who find the phpbb late are a possible loss. The atmosphere of the people here is one of your most important items apart from the workout itself (a new person could print that and never return - not what you want).
The text on the main page could have a little formatting to bring attention to some key points: example: bold (or emphasize with different font) the no expensive equipment, pay nothing, and 15 minutes items. This might be something for the rest of the text - I haven't given it much thought.
As a frequent user of this WONDERFUL site. I go to the phpbb almost exclusively. I also jump to the workout page when I forget what I'm supposed to do.
Can you guess what I teach besides Math....?
I hope you take this in the spirit I mean it. Your site and those involved (pat myself) are doing a good thing here.