As you may know I have recently returned to riding after a long absence and have forgotten everything I ever knew. How do I get across to my TB that I want extension not speed? We don't have access to trotting poles.
You may not have trotting poles, but any poles (even from the garden centre) laid on the ground will help your horse learn to stretch over them..I also agree with being very deliberate in your rising in the learning stages. Your horse will need a fair bit of balance and muscle to acheive this, so build up slowly
When you say you want 'extension' can I just clarify what you mean.
I take it if you want true extension then your horse is already working in a working trot and medium trot and you have good collection and engagement and your horse is working properly from behind into a soft outline? if this is the case poles won't help IMO, you just need to get him to sit more deeply and ask for the power from behind ensuring you go with him whether you rise or sit and don't block the shoulders. Practise a few steps at a time bring him back and try a few more, the trasitions in and out must always be good.
Or do you mean you want your horse to lengthen his steps more? if so practise sending him forward from behind, bringing him back and sending him forward, keep working on the forward-back transitions and employ the half halt when he runs but keep your leg activating his hind legs so he knows half halt isn't a just slow down aid but a balancing and wait aid. I'm rambling a bit now