Did a hard workout in heavy trainers. They weigh 35 oz together. It was 8 x 400 in 72 with 45 second rest then 4 x 200 in 32 with 30 seconds rest. The last 400s I was hitting in 67-68, and 30 for the last 200. I've ran 15:32 in the 5k for context. Then we had 60s rest. I ran an all out 400 after that in 60. What could I run in spikes?