The suggestion is to add an opportunity to customize split size/length.

The use case is interval training, running, or cycling. For this type of training, 5min splits don't suit well, as the split times are not representative. It would be nice to be able to set a pattern of splits. The pattern can be based on time and/or distance. Whenever users start training, they can select a split pattern they want to apply.

The simplest solution is adding a text field, describing a pattern of intervals by some documented syntax.

Examples:

5min;((1min;40sec)*5)2min)*3;4min - this pattern describes 5min warm up, the 3 sets separated by 2min slow tempo, each set contains 5 intervals separated by 40sec slow tempo. The result is 35 splits. Users can write in another way to avoid adjacent relaxing splits. It's up to users to decide.
1km;((300m;100m)*5)400m)*3;1km - similar pattern describing distances instead of times.
5min;((300m;30sec)*5)2min)*3;1km - combined pattern, both time and distances.

Also, you may come up with some UI to handle this.