Capacity (Ahadith 950 - 951)


Volume 2, Book 21, Number 251r:
Narrated Anas bin Malik
Once the Prophet (p.b.u.h) entered the Mosque and saw a rope hanging in between its two pillars. He said, "What is this rope?" The people said, "This rope is for Zainab who, when she feels tired, holds it (to keep standing for the prayer.)" The Prophet said, "Don't use it. Remove the rope. You should pray as long as you feel active, and when you get tired, sit down."

Volume 2, Book 21, Number 251n:
Narrated 'Aisha:
A woman from the tribe of Bani Asad was sitting with me and Allah's Apostle (p.b.u.h) came to my house and said, "Who is this?" I said, "(She is) So and so. She does not sleep at night because she is engaged in prayer." The Prophet said disapprovingly: Do (good) deeds which is within your capacity as Allah never gets tired of giving rewards till you get tired of doing good deeds."

Volume 2, Book 21, Number 252:
Narrated 'Abdullah bin 'Amr bin Al-'As:
Allah's Apostle said to me, "O 'Abdullah! Do not be like so and so who used to pray at night and then stopped the night prayer."

It's simple, really. What we tend to do is overdo stuff and get exhausted quickly. Say I got to know about the Sunnah of praying Tahajjud at night. I got so excited and motivated that I vowed to pray looooong rak`aat the whole night. I did it for one night, gave myself a tap on the back and felt good about it. Next night, again. And again. A few days later, I got so tired and exhausted that I left it completely.
There are several cons to this behavior. One of them is, that it lacks consistency. I did not continue the deed for long. I could not continue it, actually.
Narrated By 'Aisha (RA) : The Prophet (SAW) was asked, "What deeds are loved most by Allah?" He (SAW) said, "The most regular constant deeds even though they may be few." He (SAW) added, 'Don't take upon yourselves, except the deeds which are within your ability."
[Bukhari: Vol 8, Book 76, Number 472]
So take it easy. Hold your horses. :P
Whatever you do, make sure you do it continually. Quantity doesn't matter. Quality does.