My first B2B long runs, in a bonk way

Last weekend I completed my first back-to-back (B2B) long runs for the coming Sundown marathon preparation. B2B long runs are not necessary for marathon training, I just did them because I missed a few tuning up long runs for millage accumulation since I injured my left sole in the late Mar. So I planned the 21km/21km long runs in the morning and evening respectively, instead of going 30km in a single.

I'v been carried out "running on empty" training style since last June. I think it critical to my endurance training because running under a glycogen depleted mode would enhance fat burning ability. I usually did my key runs in the mornings without breakfast 3 times a week, including 2 medium runs and a long run, all 1.5hr above. The pace was easy and under MAF heart rate (<147bpm). My adaption developed well and now I would not feel hungry after the runs, even  a 3hrs run.

To make the training even hard, I decided go bonking: first run to deplete glycogen storage and 2nd run to run on tired legs. So the 1st run should be in medium intensity and 2nd run in lower intensity. At 5:30am, I carried out a progressive 21km run. After a 1mile warm up, I increased pace from 6:34/km to 5:37/km, heart rate increased from 142 to 171bpm respectively. And followed by a 1mile cool down. The run was stressful and sure glycogen used as the main energy source according to the heart rate. And I also felt quads soreness after the run. It was due to high lactate acid released during the high intensity workout, ended up with H+ accumulated in the muscle fibers.

I did not play harder and went real bonk by carrying out the second workout in the afternoon (before glycogen replenished), also because of the hot weather in Singapore. I was not worried about the bonk efficiency by doing the second in after 2 meals, because I was confident about my fat burning ability even when my glycogen storage was full, thanks to a years "running on empty" training.

I did the second run in a peaceful pace, ranging from 6:54/km to 7:12/km, heart rate from 136 to 143bpm. My gait and form were kept well regardless the morning run, except quads soreness developed into pain (might be muscle damage). After 13km I was forced to change the form, more relying on hamstring and soleus. Only by then I realized my normal gaits a more quads one rather than a traditional hamstring one. No wonder I could hardly improve speed now! I decided to train hamstring more a change my gaits later.

The B2B runs were did in 4:51 and my body was fully recovered in 3 days (quads pain only), without injury, nor bad stress. The MAF run after shown 20sec+ MAF pace improvement over the week before.

Apenix: B2B run data sheet
Run    Purpose                    Distance         Time         Avg Pace      Avg HR
1        Glycogen depletion    21.6km          2:16          6:18/km        155bpm
2        Stress adaption          21.4km          2:35          7:17/km        138bpm