Golden week, JR pass = basicaly useless?

I arrive in Tokyo on April 29, I was hoping to go to Sapporo around the 30th or 31st of may for a couple of nights. Last trip, I just turned up at the counter minutes before the train was due to leave, and the most I had to wait was an hour if the train was sold out.

Is a single person going to have trouble "winging it" by not making advanced plans during the period? It sounds like I'll be stuck in Tokyo between April 29 and May 8 if I prefer to buy tickets only half a day in advance.

Nope

I arrive in Tokyo on April 29, I was hoping to go to Sapporo around the 30th or 31st of may for a couple of nights. Last trip, I just turned up at the counter minutes before the train was due to leave, and the most I had to wait was an hour if the train was sold out.

You will definitely be able to make it to Sapporo. The question is just how simple and comfortable it is going to be.

Trains will be busy due to the Golden Week and the cherry blossoms which will likely be in full bloom in northern Tohoku and/or southern Hokkaido.

But not all of the trains will be booked out. Furthermore, note that there are trains with non-reserved seats going as far north of Morioka which you could always use without a seat reservation. Beyond Morioka, the shinkansen to Hokkaido are much less likely to be fully booked out.

Also, note that the Golden Week this year is not very favorable for local people. It is basically just two 3-day periods. As a result, it is likely to be a little less extreme than the average Golden Week.

April 29 is likely to be very busy especially in the morning and early afternoon. In the evening, it is much less busy.

April 30 is the middle day of the first 3-day period. It is likely to be a little less busy, especially if the weather happens to be bad (and less people go hanami).

May 1 is likely more busy again, but in the opposite direction. The rush is probably more busy in the afternoon than in the evening.

May 2 is a regular weekday. It is likely to be the least busy of the four mentioned days, although some Hayabusa tend to sell out especially in the mornings.

Thanks for your input uji, I feel a lot less worried about it now.

Just one question, I grew up in a regional area of Australia with only 100,000 inhabitants and my entire state has a reputation of being boring with no true nightlife/events, will I be disappointed if I only go to north/eastern Honshu and Hokkaido? I went to Osaka and Kyoto very briefly on my first trip but that was mainly to just ride the bullet train. My understanding is that the Tohoku region has far fewer people and cities than Kansai and further west.

Perhaps that's a good thing, as I'll get to experience the "real" Japan, it's just that I'm also not very appreciative of nature as half of my state is untouched forest and world heritage area.

Nature is a big draw of northern Japan, especially Hokkaido. It also scores high in onsen and rural landscapes. If that does not sound great to, you then you should consider visiting a different part of the country.

I am from a country that has lots of mountains. Some people tell me that Japan's mountains must be boring to me. However, I am fascinated by the mountains in Japan because they are volcanic, while the mountains in my home country are not. I love visiting the hell valleys and onsen of northern Japan and hike in the volcanic landscapes. To me it is amazing that the mountains are living, and mountain rivers can be hot.

How about from 2nd until 7th May 2016? is that period still consider as Golden Week? or the crowd become lesser?

The basics are explained here:
http://www.japan-guide.com/e/e2282.html

Generally speaking, the weekends are not very favorably placed. Children and most employees will have to go to school/ work on May 2 and 6, not allowing for extended holiday periods. While it will be very busy, it will probably not be as extreme as in other years.