I'm going to try two intervals sessions per week plus one long run (at weekends). I'm following the 4 x 4 model developed by the Cardiac Exercise Research Group of the Norwegian University of Science and Technology. They have produced strong evidence from research that shows a significant increase in VO2max after 6 to 12 weeks of interval training. ntnu.edu/cerg/advice
Following a warm-up of 10 minutes (I walked for 5 and jogged for a further 5) there is a fast run (at 85-95% of max HR) for 4 minutes, followed by a 3-minute walk. This is repeated 3 more times followed by a cool down.
Reviewing my numbers, I could have gone harder – my maximum HR was only 90% (I actually eased off when I saw my HR reading was 156 – I should have let it rise to 165!). My pace in the fast intervals was around/slightly below 5 min/km (scarily quick for my old legs!) with a peak of 4.06 min/km. The 1k, 2k and 3k intervals were below 6 min/km, which was an average including the 3 minute active rest at walking pace. More confirmation I suppose that Jeffing works!
I was also attempting to put into practice some of Danny Dreyer's 'Chi Running' tips, so perhaps that explains why the numbers were so surprising. I'd definitely recommend the book as a stimulating and informative read.
My cool down was a walk home to change into my Nike Frees then back to the field where I jogged another 1k, although looking at my Garmin stats, 5 mins 40 secs pace (HR in 'aerobic' range) isn't exactly jogging! I can confirm that midfoot striking and following the Chi Running principles appears to enable faster and economical running!
Total distance was 6.58km, in 42 minutes, average moving pace 6.24 min/km. So I'm feeling really pleased with my experiment, very positive about the research I've done and quite excited about my potential (and blown away by the thought I might achieve a 5k time between 25 and 20 minutes!). But I'm also a little frustrated that all the years of sport at school never uncovered any athletic ability! Onwards and upwards – keep running and keep enjoying it!