Is it permissible to delay the Isha prayer for an hour or two past its time for the purpose of exercising, and if it is not permissible, what is the sin of one who does so?

A Muslim must perform congregational prayer on time, and it is not permissible for him to prioritize worldly matters over it except for an excuse. The Prophet (peace and blessings be upon him) made congregational prayer obligatory even for the blind. If a Muslim prays alone on time, his prayer is valid, but he misses out on a great reward by going against the Sunnah of the Prophet (peace and blessings be upon him). Therefore, he must repent and be keen on performing congregational prayer.