My ukemi started to improve drastically when it occured to me that the role of uke is where at least half of the 'art' of aikido is learnt. Learning how to follow the exercise from start to finish whilst maintaining connection and complete coordination throughout is in my opinion the best place to learn how to improve your aikido.
Jun's 7 year old post above sums it up neatly and is showing no signs of age
The speed of any technique/exercise should be governed by uke, so uke should only go as quickly as they can safely accomodate themselves. If nage is going faster than uke then they are doing something other than aikido.
Take things slowly and steadily, there is a long way to go