In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Indeed, Allaah [The Mighty and Majestic] placed strength of a believer in the heart and not the other limbs. Have you not seen that a weak old man fasting when the sun is very hot and stands up at night in prayer, whilst a young man is unable to do so?! [Ref 1]
Imaam Ibnul Qayyim [rahimahullaah] said:
There are two (types) of strengths within the soul: The strength of abstinence and the strength of courage. The reality of patience is that one makes the strength of courageousness a means of action in what benefits him and the strength of abstinence a means of action in refraining from what is harmful to him. Among the people, there is one whose patience in carrying out what benefits him and being firm upon it is stronger than his patience in refraining from what is harmful to him; so he exercises patience in facing difficulty whilst carrying out acts of obedience, but he does not exercise patience in refraining from urges towards what he is forbidden. And among them, there is one whose patience in refraining from things that are in opposition to the religion is stronger than his patience in carrying out acts of obedience, and among them there is one who does not exercise patience in both affairs. The best of people are those who fulfil both affairs. Many people patiently bear the hardship of praying Tahajjud during hot and cold nights, and the difficulty in fasting; but they do not exercise patience in refraining from looking at forbidden things. Many people exercise patience in refraining from looking at the forbidden things and inclination towards [forbidden] images, but they do not exercise patience in commanding good and forbidding evil..[Ref 2]
[Ref 1: Hilyatul Awliyaa 1/480]
[Ref 2: An Excerpt from Uddatus Saabireen. page 37. Slightly paraphrased]