There are 2 types of exercises which you can apply here.
With regard to physical exercise is that you pratice sitting in a particular posture gradually increasing the duration of your sitting.
In meditation pratice by S.N.Goenka this is developed as part of developing your resolve as well as the ability to sit for a long hours.
Another p±ram² is adhiµµh±na—strong determination. When one starts a Vipassana course, one makes a determination to remain for the entire period of the course. One resolves to follow the precepts, the rule of silence, all the discipline of the course. After the introduction of the technique of Vipassana itself, one makes a strong determination to meditate for the entire hour during each group sitting without opening eyes, hands or legs. At a later stage on the path, this p±ram² will be very important; when coming close to the final goal, one must be ready to sit without break until reaching liberation. For this purpose it is necessary to develop strong determination.
Source: The Discourse Summaries by S.N.Goenka
In order to be able to sit for long hours you have to experience lightness in body and mind. You have to do this by actively releasing tensions by looking at gross solidified sensation part by part and if you have a free flow of subtle sensation in your body sweeping through them opens up the areas which are either numb or there is gross solidified sensations.
Also one way that the body will get lighter is to have sustained concentration on an object, i.e., you first bring your mind to the object (e.g. breath) then you continuously repeat the same metal redirection of the mind to the object regardless of the mind wandering away. Also walking meditation is able to develop stable concentration which intern will create lightness in the body.
For a more technical answer please see: Having trouble meditating long sessions (Physical pain)
Also I like to add the following note. Though in the 10 day meditation taught by S.N.Goenka there is no formal time allocated for walking meditation, walking is considered an antidote to drowsiness and sleepiness in the spirit of the following Sutta.
... Bhikshus, what more should be done? Bhikshus, you should train yourselves thus, We will devote ourselves to wakefulness.
During the day, while walking to and fro and sitting down, we will purify the mind of obstructions.
During the first watch of the night, while walking to and fro and sitting down, we will purify
the mind of obstructions (avarana synonym for nīvarana).
During the middle watch of the night, we will, after mentally noting the time for rising,
and fully aware lie down, lion-like on our right side, one foot placed on the other. ...
Source: Maha Assa,pura Sutta. Also see footnote 30. The context here is staying awake and fighting drowsiness.
If, Moggallāna, that drowsiness still would not go away, then, Moggallāna,
you should, perceiving before and after, be resolute in walking back and forth, with the senses
turned inward, with the mind not straying outward. It is possible that when you do so, that drowsiness would go away.
Source: Pacalā Sutta. The context here is staying awake and fighting drowsiness.
These suttas only advocate this as an antidote to sleepiness and as a means to keep awake than any other form of pratice. This how S.N.Goenka teachers. This is inline with the Suttas.
Another enemy is laziness, drowsiness. All night you slept soundly, and yet when you sit to meditate, you feel very sleepy. This sleepiness is caused by your mental impurities, which would be driven out by the practice of Vipassana, and which therefore try to stop you from meditating. You must fight to prevent this enemy from overpowering you. Breathe slightly hard, or else get up, sprinkle cold water on your eyes, or walk a little, and then sit again.
Source: The Discourse Summaries
NB: Added this note as to there is no misunderstanding about this pratice, but not to condemn any other pratice.