Does rucking get easier?

Hey guys, I'm training for my Tough in DC and wanted to know if rucking 45lbs get easier on the shoulders after you do it more? Does the body get to the point where it just feels that 45lbs on your back is no longer there? This is a question to all those who already can ruck for long durations. I notice the area where the ruck sits on the top of your shoulders starts to ache and I start feeling the suck about 1 hour into my rucks. I switch from using the chest sternum strap and not during the ruck and I find it makes some difference. I'm just wondering how I'll be able to do this for 12 hours + some log or team weight.