Recognising that your horse may not have the strength to perform the work you are asking for is key to ensuring he remains sound and willing throughout his Dressage career.
Consider this: If you embarked on a program of stretching, joined a yoga or pilates class or even joined a gym how would you feel after your first session? Perhaps a little sore, perhaps a little tired. How long could you do and how often would you need to go before it became easier and enjoyable?
What would happen if you jumped in at the deep end and did the ‘body blast’ class? My guess is you would be laid up for a few days and probably wouldn’t want to go again.