The known variables are the run duration, walk duration, and average pace. The unknowns are the run speed and walking speed. So I used several different walking speeds as inputs to generate the possible run speeds being used. Most people walk around a 16-17 min/mile. But I know people who walk faster and slower. I personally walk at a 19-20 min/mile pace normally.
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If the walking pace ranges from a 12 min/mile to a 20 min/mile, then the run pace ranges from a 7:40-8:00 min/mile for the 3:40 M and 7:54-8:20 min/mile for the 3:50 M.
3:40 M race pace equivalency
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So for a 3:40 M runner, a 7:40-8:00 min/mile is roughly 10k to HM pace for 3 min with a 30 second rest. Or something like 620m (0.385 miles) intervals at 10k to HM pace.
3:50 M race pace equivalency
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So for a 3:50 M runner, a 7:54-8:20 min/mile is roughly 8k to 11 mile race pace for 2.5 min with a 30 second rest. Or something like 500m (0.31 miles) intervals at 8k to 11 mile pace.
The difference between the two is that the 3:50 runner will have to run slightly faster relative to their own fitness level for a slightly shorter time. If a 3:50 M runner tried to do the 3:40 M pace group, then they would be doing roughly 5k-10k pace for those intervals instead of 10k-HM pace.
Interval distance wise the 3:40 is close to 1.5 miles after 4 sets (1.542 miles) and 3:50 is close to 1.5 miles after 5 sets (1.552 miles). So you might consider trying to do an every other type training run to see which methodology you feel most comfortable with. Do 1.5 miles at 8:24 pace, and then 1.5 miles at run/walk, and then 1.5 miles at 8:24 pace, and then 1.5 miles at run/walk. See how they feel differently and whether one feels more sustainable.
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If the walking pace ranges from a 12 min/mile to a 20 min/mile, then the run pace ranges from a 8:52-9:50 min/mile for the 4:30 M.
4:30 M race pace equivalency
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So for a 4:30 M runner, a 8:52-9:50 min/mile is roughly 4k to 20k race pace for 90 sec with a 30 second rest. Or something like 262m (0.163 miles) intervals at 4k to 20k pace. Because of the shorter ratio vs the 3:40/3:50 groups above, the walking pace actually used by the pacers becomes more important as to what the actual run pace would end up being. If they walk slower, then it dramatically increases the necessary run pace.
Six intervals gets you to 0.978 miles.