You could start working with the rope as an assist like I did in the video...arms up and curl assist as you sit up. Gradually keep the arms lower as you pull to assist the situp and finally keep the fists and rope on the floor...like the video. You could probably do it in a few months. Just slowly progress. The goal is the unanchored situp...the purpose is to gain the strength to do the situp.
Re: rock climbing...I did that as a 16-year-old, long before tree work and I rock climbed ever since, a few times a year, nothing real intensive. My introduction to rock climbing was a week-long course taught by Army Rangers on Mt. Yonah in N. Georgia. It was one week of a 4-week survival course (called Operation Upstream)that was patterned after Outward Bound. The other weeks were basic camping, survival skills, first aid, orienteering, etc. for the first week. The second week was mountain training. Third week was water survival... self-rescue (drownproofing), lifesaving (rescuing others), group survival. We simulated a group survival situation where a plane went down and we had about 20 people to keep alive until help arrived. It was a COLD Lake Conasauge in N. GA and we had good swimmers, bad swimmers and NO swimmers. We had to do tired swimmer carries, rotate out keeping the drowners afloat and deal with hypothermia at the same time. We probably spent 3 hours in the water. Skinny me like to froze to death. I am a good swimmer and already was a lifeguard before the course...swimming was no problem. Dealing with the cold was. It was like SEAL cold water training. We could give up and get in a support boat (they monitored us during the swim) but that was called "crash and burn". It got you a pretty bad reputation. We learned better how our minds worked with the suffering. Quite an experience for 16-year-old city boys.
The fourth week was a 3-day solo and graduation -- we were scattered out through the Cohutta National Forest, probably a 1/4 mile apart and had to set up a camp and either eat or fast for 3 days. To eat we had to trap/fish, etc. I was able to catch a few trout and they were awesomely good. I learned that 3 days with no human interaction is a LONG time. I was glad to get back around folks...even the ones I did not like. And the last part of the 4th week was graduation...competitions on the obstacle course, a 10 mile race through the mountains, etc. We exercised everyday at daybreak, a short session and then broke for chow. We ate C-rations the whole month. After bfast we did a longer exercise session and then classes, practical exercises -- e.g., learning to build a stretcher and then they took us about a mile away and declared one of us a casualty. We had to build a stretcher on site...usually a ridge or down in a creek bottom...and carry the victim back to camp...after we treated the injury. Each team was about 10 boys. That's enough...pretty big derail here.