Why not make the 100 mile program a duplicate of the the mileage medallion program that AERC already has in place. Base the 100 mile award strictly on the number of 100s completed - no mileage requirement. It keeps it simple for book keeping purposes and keeps the cost down if only awarding medallions/certificates for the first two levels. Medallions could be given at 2- 100s 5- 100s 10- 100s and continue in increments of 5. Horses reaching the 10 100s or 1000 miles of one day 100s would receive a blanket.
I guess you could use bronze, silver, gold, platinum, diamond to indicate the levels but what would you call a horse with 20+ 100 other then "incredible"?