trainig heart-rate zone

[jovana]

Please,tell me which is the best heart-rate zone for training in percentage? i spend most time in 80-90% of mhr when i am on hilly roads(gradient of descent 10% and less).
can i improve my shape and endurance in this zone,or i should stick in 70-80% of my mhr.
i spend a lot of time above 80% of mhr on a flat roads, too.i started to ride 2weeks ago, and have had 8 trinings since then.
how many training days are needed for some visible improvment(monitoring heart work and speed).
my cadence is about 100 (less or more for a few rpm),and my average speed is about 19ml/h.today i had hilly part about 3 miles with a gradient of descent 8 %,and have had average speed 13.15 ml/h.can u tell me is it to bad,or what.(almost all the way i was at 90-95% of my mhr)