Yes, Carrie started the guides with 34 5-day units but as her kids got older she realized that more flexibility was needed for outside activities, etc. so approximately 5 days of work is scheduled in 4 for Preparing on up. There is also one extra (35th) unit in those guides (compared to the younger ones) so LHFHG to Bigger have 170 days while Preparing through High School have 140.
When scheduling guides from both groups (let's call them the lower and upper guides for ease) you can handle it a few ways:
~ Doing 5 day and 4 day weeks combined at full speed: a younger child could do Unit 1 (M-F) while an older one does Unit 1 (4 days), using the fifth day (take your pick) for catching up unfinished work, music lessons, sports or any other interests.
~ Doing only 4 day weeks: while the older child does 1 unit per week (with the fifth day for catch-up or outside interests), the younger child only does 4 days as well (making the fifth day a day "off" of academics for the whole family, freeing it up to do outside activities, co-op, lessons, shopping, etc.), which means that the second week the younger child would do Unit 1 Day 5, then Unit 2 Days 1-3, etc. They would slowly get "further behind" the older siblings, if looking at weeks, but it makes family life go a bit smoother. This way, a guide would take a little more than a school year.
~ A combination. Just do an HOD Day every day you "do school" and don't worry about whether they're lined up or not. Maybe some days you'll combine a few Bible stories and do an extra activity in LHFHG to do 3 days in 2 days. Maybe your oldest will need a catch-up day. Don't worry so long as you get about 4 Days done per week (on average) and they keep progressing in their skills.
FWIW, this last year I found out that I really like having each child switch guides at different times of the year. Strange circumstances have caused each of my children to finish at different times and that has allowed me extra time to finish up each guide well (saving things to a portfolio, not trying to rush to finish by a certain date, etc.) and to spend extra time with them as they start each guide half speed for a couple of weeks (rather than trying to start 3 or 4 guides at the same time). I used to be a "one guide per year, finish up everything within a few weeks and definitely by the end of June" homeschooler, but am slowly (after 10+ years homeschooling) learning to relax and enjoy the journey. Each of their paths will be different anyway, so I might as well embrace those differences from day one!

This year my youngest in HOD started September in about Unit 3 of LHTH, my Beyond guy still had about 3 units of LHFHG to finish before starting his new guide, my 10yo started his guide at the beginning of the school year, my DD started the school year in Unit 11 and my oldest started in Unit 25! It was a much more relaxed start with some kids going back to what was familiar than the previous year, starting 4 kids in 3 HOD guides at once, for our first year with HOD.
Blessings as you continue to wrap your mind around what HOD looks like and how it will play out in your family!