I am not a big supplement fan and here is why. Last year, Mens Health magazine did a review of the supplement industry. It harsh review of a shady industry, and Mens Health takes a lot of advertising money from the people the article went after. Most of the stuff being sold is really untested and unproven. Ever notice that a supplement does not stay on the market long? It's because most people figure out that the stuff does not work and then move onto something else in search of that quick fix. Most of the stuff sold is just a renamed version of something from the past. GNC is one of the worst offenders when it comes to renaming. The claims made by the mfg's are never tested or proven, this is why any statements made carry the "not validated by the FDA" type of warning. The industry is not regulated and its really the "science" wild wild west when it comes to claims and facts. The quality control on these products is totally lacking. There is no certification that what is on the label is what really goes in the bottle. This is stuff I would rather not put in my system.
You mentioned being wrecked the day after working out. This is going to be due to diet and fitness level. If working out is new to you, yes you will be sore, yes you will be tired, and yes you will feel like hell. You also mentioned having trouble keeping a 14 min/mile pace on the treadmill. That pace is just faster than walking. My guess is that your fitness level was rather low, armchair athlete low - I'm not knocking you, after all you did make the decision to get fit and that is always a good move. So, struggling in the beginning would be normal. A few weeks at the gym and you would see those 14 min/miles drop to 9 minute miles as your overall fitness level increases. Hard work and time are the only things that are going to increase fitness levels and muscle mass. I know this country has a love affair with pills and magic fixes, we really are a pill nation. The only magic pills that will boost performance and muscle building are not available at GNC - you would have to see Barry Bonds, Mark MaGuire, Sammy Sosa, and Rodger Clemens as they will tell you what to get and who is brewing it up. Its the really good illegal stuff like HGH and clenbuterol. Next, all the stuff you see on TV is marketing hype. The Bowflex guy with the unreal 6 pack abs does not work out just 20 minutes a day on a bowflex. It took that guy years of hard work, strict diet, and self control to get chiseled like that. Bowflex is a great system but the results claimed in the commercial are far from what someone will see in just 6-8 weeks. Try seeing a registered dietitian and get some sessions with a professional sports trainer or kineisiologist. A Vo2 max test would be helpful as well. This test will determine your potential to get fit as well as your natural fitness level. Genetics are involved, not everyone will be world class with the right training - this is a whole different subject for another post.
I would be interested to know what your pre-working out days BMI was, height and weight, as well as your typical daily diet.