edzo's plan of sprint/long/hill intervals will work without needing a HRM. You can do them based upon perceived effort and exertion. As you get stronger and more fit, that same exertion-level will have you going faster that's all, but you're still working as hard relative to your max. While the distances and efforts will vary, you pretty much want to be completely maxed out by the end of them. No need for a HRM to tell you that you're completely 100% spent! If you have anything left to give, you didn't do the interval hard enough.
Be careful with cadence though, you should be spinning fast, faster than you normally use for a steady pace endurance ride. Aim for 95-115 rpms, maybe calibrate with someone else who's got cadence on their computer.