There's no real reason why you can't get stronger at riding the hills without riding them during the week.
Some of the best climbers I know do the majority of their weekday training on a trainer. And there is no reason why you can't do the same on the river bank.
Like @Musashi13 says, 20, 15, 10 minute intervals are good. You should be going as hard as you can for the duration. A rest interval of 5 mins. At first do 2 sets. Build to 3. I find 4 sets, eg. 4x15 mins, is about as far as I can go.
Without a power meter, you can use HR for your pacing.
Look at the data from your best climbs. What was your steady HR towards the end? What was your cadence?
Try to mimic this on the flats. Build to a target HR. Your target cadence may be 5 to 10rpm higher.
If you have a specific target you want to hit, I recommend chatting with @Doug3 who may be able to help you plan a progressive training schedule.
Thanks for all the replies btw, especially to @theBlob , @Musashi13 and @andywood . Some interesting stuff there. I'll have to try some intervals. Will be interesting to see how effective they are on the slopes of Shiraishi.