We are planning a trip to Japan this spring. We want to walk the Kumano Kodo, has anyone done this? What other suggestions do you have for must sees?

Hi there, I spent three 3.5 days walking the Kumano Kodo back in June. It was rainy and parts of the trail were very slippery, especially on the downhill sections (a lot of the trail was paved with river stone and sometimes it was like walking on ice). If you go in the later spring, you may encounter similar conditions. The Kumano Travel site is a great resource for planning your walk. Personally, I stayed at two minshukus (bed and breakfast) and my last night was a homestay. Stop at the visitor centre in Kii-Tanabe to get a booklet for collecting stamps along the route. It's a good souvenir and if you do the Camino Santiago in Spain, you can get a Dual Pilgrim status as these are "sister" trails. The visitor centre can also check you in and confirm your reservations.
This was my journey:
1) Arrived in Kii-Tanabe and spent the night
[]Day 1
2) bus to Takajiri-Oji
3) trekked from Takajiri-Oji to Chikatsuyu-Oji (13km)
4) bus to Yunomine Onsen, stayed overnight
[Day 2]
5) bus to Hongu Taisha
6) explored Hongu shrine and visitor centre, then bus to Shimoji-bashi
7) trek Ukagawa trailhead to Koguchi (13km), overnight
[Day 3]
trek Koguchi to Nachi-taisha (14km)
9) explored Nachi shrine and Nachi-no-otaki waterfall (tallest in japan)
10) trek from Nachi-taisha along Daimonzaka path to homestay (2km)
[Day 4]
11) bus to Kii-Katsuura and then train to Osaka
* I only went to 2 of the 3 grand shrines.
** I was crunched for time and bused about 16km in the middle section.
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Kumano Travel
https://www.kumano-travel.com
sample itineraries
http://www.tb-kumano.jp/en/itineraries/list/
Kumano Tourism Youtube Channel
https://www.youtube.com/channel/UC3JnPpdDDfAAsYrenCUx0ug
Other helpful sites.
https://japancheapo.com/entertainment/hiking-kumano-kodo/
https://boutiquejapan.com/introduction-kumano-kodo/
https://www.japan-guide.com/e/e4952.html
https://www.shinguu.jp/en/pdfmap
http://www.andyhuphoto.com/kumano-kodo-trekking-guide/
re

Japan is a def bucket list!

Ha! Literally doing it right now! If you are feeling fit, you can explore options like the tour group, WalkJapan. But we are crossing paths with a few souls who are solo-ing it. You probably don't want to walk the entire trail... Doing the 3 big temples is most important! But of the 7 days we spent walking the trails, 2 were just strenuous hikes through the beautiful forests.
Big tip from me is get bug repellent before you go! I didn't and the bugs have played havoc on me (58 bites and counting...). They mainly nibble in the accommodations though and don't seem to mind rustic accommodation or 3 star ones...
It's also VERY humid unless you do it in October to April. You will need some rain gear and if your footing isn't very good, some good walking sticks. Hiking boots (I'd say at least half ankle) are a must for stability as there are lots of slippery rocks.

Sally when is the best time of year to do this? Also, how strenuous is it?

I'd say mid-late October or march-April (which conincides either with the fall colouring or the cherry. blossoms...) Summer just has crippling humidity (which. I don't recall when I was here hiking in April).
It is very strenuous... Yesterday we walked 17km and did 1187m of elevation gain over 3 hills (800m ascent, 200m and 100+ m ascent...). We have been averaging 13 km per day with average 800-1000m total elevation gain. Always a steep part...some parts are nicer as you contour across mountains... If you are used to hiking the BC mountains, you'll be fine... If you are a bit soft (like me), you will find it hard going.
But always good to know the Japanese are adaptable... After 2 days hiking, from Hongu Temple, we discovered that the Japanese often take a boat from the temple at Shingu to Hongu, up the Kumano River. So don't feel you have to walk the entire trail - that is nowadays a tourist thing. For the Japanese, it's about visiting the 3 main temples/shrines which represent before you were born, while you are alive and after you die ..
Do start a shuinchō book though! (You can buy them at any temple) These beautiful shuin stamps cost 300¥ at each temple/shrine and are a beautiful and unique reminder of your trip. And you make sure the book gets cremated with you so that you will be happy in your afterlife ..