I have just returned home from taking the PFI class in Ft. Lauderdale. On Erin's advice, I took the intermediate and advanced classes back to back, as I wanted more diving than just the 4-day intermediate class.
Before taking these classes my deepest dive was 42 feet and probably around 35 or 40 seconds. (I was wearing a depth gauge but no timer.) My best static apnea was 3:21. Erin told me they could get me to 75 feet, which I dismissed as hyperbole, but I was confident I could get to 50 feet, and had hopes that maybe they could get me to 60. I "just knew" that below that was unrealistic for me.
The classes were amazing. We learned dive physiology, dive psychology, and dive physics, some of which I already knew, but much of which was new to me. We learned dive safety, all of which was new to me. We learned warm-up techniques, and diving techniques, from entry to equalizing, to finning, to weighting, to recovery breathing after the dive. We did static and dynamic apnea in the pool, we practiced rescue scenarios, and we learned exercises for warm-up, and for long-term improvement of lung flexibility and breath-hold time.
During the intermediate class I achieved a 20-meter dive (66 feet) with a dive time of 58 seconds. That was 6 feet deeper than my goal, and within 2 seconds of my time goal. I also managed a 3:45 static.
During the advanced class I managed a 4:15 static, and I achieved an incredible (incredible for me) dive of 27.5 meters (90 feet) with a dive time of 1:02.
I am not an athlete. I've never been any good at sports. I jog for 30 to 40 minutes at a 13:20 pace (any slower and I'd be walking rather than jogging). I didn't really belong in the advanced class. I was the only one in that class who had never been to 100 feet. And in fact I skipped some of the advanced rescue practices because they were beyond my ability. But I still learned a lot, and I am a much better and safer diver, and will be a better buddy, for having taken the class.
I highly recommend PFI for classes at whatever level you are at. If you don't know, they'll help you figure it out. Reading this thread, Erin doesn't seem to be posting here any more. But their phone number is on their web site. They are very friendly, very positive, and they know what they are doing. Very importantly for me, an instructor accompanied me on every target dive I made, and during the intermediate course, on every warm-up dive as well. (By the last couple of days of the advanced class, I was comfortable doing my warm-up pull-downs with only surface safety.)
I still find it hard to believe that I swam down to 90 feet one one breath of air, and made it back to the surface unaided. (Accompanied, of course, by Erin on that dive, as by an instructor on every dive, but entirely under my own power and on my own one breath of air.) That's more than double my previous best depth, and roughly 50% longer than my previous longest dive time. My new goal is to increase my dive time to 1:30, which I think is within the realm of possibility, with the breath-hold exercises they gave us. (O2 and CO2 tables, which are also available elsewhere, though I would recommend understanding what you're doing before you try them, as you can black out if you do them improperly.)