Shetland Driving Question

Our stocky but...sedate ;) 37" takes 20 stone plus either 2 wheeler or 75KG 4 wheeler for driving trials. Most take 2 small adults plus carriages around the 100kg mark at driving trials...
 
I have bought her a lightweight trap and I was going to try buy her a webbing harness because I thought it would be lighter for her than a leather one, I only want to toddle round the village with her but there are a lot of hills round here so I thought maybe I should get off and walk up the hills, I weigh about 8 and a half stone, I wanted to take my sister out with me too and she weighs about the same.
 
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