PBP is not simply a 1200 Km long cycling event. It also has 12,000 metres of elevation gain. The route is full of hill climbs with "gentle" slopes. There is no flat segment or stage in PBP. They will all have hill climbs. The climbs range from 500 metres to 3 Km long and the slope will vary from 1% to 6% but the average gradient will be about 3% for most climbs. You should ideally find a hill close to where you live (preferably matching the above stats) and train for climbs riding up and down that hill till you get better at climbing.
For base or endurance training, you should ride in power zone 2 or heart rate zone 2-3 for most of your training rides except for the day you are doing HIIT. Slowly build up your weekly training hours. About 80% to 90% of your weekly training hours should be in this endurance zone.
Participate in brevets that offer more elevation gain. Travel to other cities if your local region is mostly flat terrain.
You can train indoors using digital platforms like Zwift or TrainerRoad if you are crunched for time or if you live in a mostly flat region. However, indoor training should still be supplemented with outdoor hill training so you improve your bike handling skills on actual hilly terrain.
For more information, advice and resources on preparing for PBP, please check out Audax India's useful resources page for PBP at: https://www.audaxindia.in/useful-resources.php