OK -
You start off as the race gets going - the traditional phrase is "And tbey're under Starter's Orders ... and they're off!". The Starter is the guy who gets them all into line, or sess that they are in stalls, depending if it's Flat Racing, or over Jumps (fences). He then pulls a switch to open the stalls, or raise the wire, and the jockeys start their horses off, and the race is under way.
The commentator usually starts by going through the horses by name, from first to last - something like
"And it's Andy Hughes out in front, with Christy pressing hard in second place, and Shilpa's Gorgeous challenging in third place, with .... and so on through all the horses, finishing with ...and it's Jade's-a-Chav bringing up the rear..."
After that, the commentator concentrates on the first four or five horses, and advises which one is doing well, which one is getting tired -
"And as they past the half-way marker it;s still Andy Hughes out in front by some ten lengths now from Christy who is still second but tiring, and being challenged by Shilpa's Gorgeous who is running strongly on the outside ... and the back marker is still Jade's-a-Chav who may well be pulled up...."
Towards the end, say three or four furlongs from the end, each horse gets quicker, if they can, and the commentator gets more excited, and talks a lot faster because it increases the exciting atmosphere -
" ... and Andy Hughes is drawing away from Christy, but Shilpa's Gorgeous has made a late challenge and has stormed into second place, and she's trying to get on terms with Andy Hughes, and it's neck and neck, and at the line, it's just Andy Hughes, from Shilpa's Gorgeous, with Christy running on in third ... (and so on naming all the horses as they come in) ... and Jade's-a-Chave failed to finish and was pulled up ..."
Right then - terms -