[run] calculates pace after time start even if GPS logging starts later

### Affected hardware version

Bangle 2

### Your firmware version

2v12

### The bug

The pace should be calculated from the distance and the time since GPS recording has started.

Observed behaviour: GPS locked after 30 minutes into the run, after one kilometer the pace was 36 minutes per kilometer or so.
This looked like the time since start of the run was divided by the distance covered since the GPS had a lock.

Expected behaviour: pace would be calculated from the distance and the time since the gps lock has started, not the full time of the run. (It is okay do displace the full time of the run, which is useful if the GPS is not working.)

Start with the [run] app's GPS logging and pace calculation, comparing the time origin used for GPS distance with the overall run start time. Reproduce the delayed-GPS scenario and verify that pace uses elapsed time since GPS recording began while still displaying the full run duration if GPS is unavailable.

Written by the indexing model from the issue text.