This is all abit subjective really. Here in Carmarthenshire most of the land lies above 500 ft and is riddled with steep valleys, The secondary roads tend to go up and over,whilst main roads tend to follow the valleys. Unlike most hiily or moutainous counties the road network is extensive ( not exactly sure why this is,could be something to do with the many tiny villages and small farms). Having ridden in most counties in mainland Britain (lincolnshire being the exception
) the rate at which climbs and descents come and go is mind boggling. Anyway I digress. I would consider a ride with anything up to 1000ft per 10 miles as flat 1000ft to 3000ft as lumpy Over 3000 ft hilly. Needless to say there are not many flat rides and most would be in the lumpy/hilly category. Within a 5 mile radius of my house I can think of over a dozen climbs over 3/4 mile at an average gradient of over 15 % 4 shorter at 20% and 2 at 25% That doesn t include ones I dont ride regularly. My round the block training lap is 3miles long 800ft of climbing not in the hilly category
It all depends on your interpretation and what your used to. Repetition I think is the key ,if you are continually climbing and descending with no flat respite and no time to recover( altitude is immaterial IMO as long as the gradient is tough and long enough ) thats a hilly ride.