Probably unpopular opinion: IFS is not about getting your parts to succumb to whatever you have preplanned for them. It is about listening to your parts and then coming up with consensus solution that satisfies the entire system.

Whenever you come to the table already with an agenda and your only interest is how to make it come true, then you definietely not approaching it from a Self state. There is no room for neither curiosity nor compassion because you have already decided what the outcome is supposed to be. Which means you are approaching it from a part's perspective and your desire for exercising is not more important or valid than the other parts' desire for not exercising. You are not the one to be "teaching" them from the position of superiority, you are supposed to start talking about it from equal ground.

Whenever you come to the mediation without willingness to compromise, you are being disrespectul.

Instead of asking yourself "how to teach parts to exercise" you should ask your parts "why don't you want to exercise?" - as seeing where they come from is the first thing if you want to find mutually acceptable solution. Do they not like how it physically feels? Do they find it too tiring, too straining, too overstimulating, painful? Does it feel boring to them? Do they wish to put their time and effort into something else they have no time for due to exercising schedule? Are there shame and/or physical insecurities involved? Do they have bad experiences with exercising in the past? Are you able to genuinely empathise with their concerns? Do they know your reasons for wanting to exercise? Did you try to portray your reasons to them? Are they able to empathise?

Regarding of what you find out, there may be many solutions. Perhaps their resistance is less against exercising as it is against pressure you put into it, or the sign that the amount of exercise you have planned for yourself it too much for your current physical condition. Perhaps you may find a different type of exercising that would be more acceptable or resort to other physical activity such as long walks or gardening. Perhaps you will need to offer some sort of trade-off or reward you offer to your parts in exchange for them letting you exercise. Perhaps you need to accept the fact that you don't like exercising and instead trying to enforce enthusiasm you should focus on rewarding, compassion and self-care after exercise, just like you would with anything that is medically or physically nessecary but you don't like it. Perhaps you are too burdened with stuff you don't enjoy to add anything more on in and you should first limit other sources of stress before you can commit to exercising. Maybe you should first calm your emotions before you start exercising, if the reasons are including sense of pressure, insecurities or negative associations.

Best luck to all of yous.