Apple Watch not counting my steps correctly

I’ve been doing an American Heart Association walk - a 3 mile walk; for each mile the video tells me I have now walked one mile - my watch says.76; at 2 miles my watch says1.53; at 3 miles my watch says I’ve now walked 2.17 miles. I’ve tried this repeatedly, sometimes I can get it to .8, 1.6, and 1.4 - never the 3 miles. I’ve reset my watch, have location and distance services turned on in my phone settings - what else can I do?