Why does Rashi say the pasuk is referring specifically to ameilus? Maybe the reward promised is for stam learning?
To understand what Rashi means we need to first understand what the chiddush is in the concept of ameilus baTorah. Doesn’t every accomplishment require ameilus? If you want to be a doctor, you have to work hard to get through medical school; if you want to be a lawyer, you have to work hard to get through law school. If you want to be successful in learning, it takes work – peshita, mai kah mashma lan?
Yesh lachkor: is ameilus a means to the end of knowing Torah, or is Torah a vehicle that Hashem gave us to bring out a certain type of ameilus? Or to put it another way, when a person is engaged in the shakla v’yerya give and take of learning, where every answer inspires a new, deeper question, and every question brings an answer that lends greater clarity, are the questions just a means to unravel the sugya and get to the answers, or are answers just the foundation upon which deeper, more probing questions can be built?
The answer is black on white in Chazal: “Adam l’ama yulad” – man was created for the sake of work, which the gemara (Sanhedrin 99b) darshens the pasuk as referring specifically to ameilus in Torah. Ameilus in Torah is the tachlis, not just the means.
In all other disciplines, the work is just a way of attaining mastery of some skill or subject matter. When it comes to Torah study, the the goal is to be immersed in questions; the struggle in learning itself is the goal, not arriving at conclusions.
Explains the Sefas Emes, when one is learning with ameilus, at every stage the knowledge that is yet to come, that one anticipates and is working toward, is a mystery relative to where one is holding at that moment. There is always a “bechukosai,” something yet to be understood that sits just beyond the horizon. “Im bechukosai teileichu” – if your learning is focused on looking forward to the next unknown, if you cherish the mystery of questions for their own sake and not just as a means to get to answers, then this is ameilus baTorah.
"Zos chukas haTorah" -- not chukas haparah or chukas hataharah. Chok, the pursuit of the next unknown, is the engine that drives all learning.