My understanding is that this part of the forum is for staying healthy, even sober life is about the part after/without/inbetween drug use. Withdrawals are still part of drug use technically so OD (Other Drugs) seems like the forum for it.
I've been through w/d from a habit of oxy then raw opium, the withdrawals did make me feel sick as a dog but with the choice made and the intention set that you cannot and won't keep going on knowing that it leads to misery - being sick for a number of days is a relatively minor thing to go through compared to some other misery. It's comparable to choosing to get the flu in a way - if you have the right values in mind a person can definitely do it, then you basically want to as a sacrifice.
From what I read I am not the only one to consider opiate withdrawals doable, it's the dealing with the period afterwards that is the real challenge - issues with mood, energy and motivation. Question is what made/makes you relapse and what or who supports you in place of the drugs when you are not on them anymore? If nothing changes in your life, withdrawing is winning a battle but not the war. It's the war that it's all about. If you figure things out on that front, the number of days of withdrawal to go through should be inconsequential.
Expect to be worn out and deeply tired from it, if not immediately then at some point after the withdrawals - then what it comes down to is how good of a safety net you have to support you. If you have absolutely nothing, then realistically it is a lot to ask from a person not to turn to the drugs again. The statistics don't lie about it, and being realistic is important. Not to demotivate you - by all means I strongly encourage you to arrange some form of support, planning ahead - to give yourself an actually good chance.
Exercise is an absolutely incredible way to train your body and mind making you not only physically stronger but also resilient to say no to the 'sins' and desires of your unhealthy habits.