Not exactly what you’re looking for, but you can use the free website https://runalyze.com which syncs with your Garmin Connect account. You can view your activities with arbitrary laps by distance or time. Or if you sync with Strava, it shows you 1 mile/km splits, in addition to the laps contained in the activity — but there’s far less data and you can’t customize the splits.
Bump... I completely agree. It seems very strange that this functionality does not exist already. It would be VERY useful for post analysis of your activity.
