Thought I'd share this for anyone questioning whether this programme works in terms of improving your fitness (not that I think many people here do!). I have an annual health assessment through work which includes a 'bike test' to assess your cardiovascular performance, etc.
Last year, I reached the 85% of estimated maximum heart-rate that concludes the test in about 6 minutes and my VO2 Max score (aerobic capacity) was a low average (for my age group) 25.7. This year it took me 7.5 minutes and my score had increased to a high 32.4. Still huge room for further improvement of course, but I found it so encouraging to see so graphically the positive impact of just 5 1/2 weeks of C25K.