For "learn at your own pace" trainings, the key to ensuring people take the entire thing and do the labs is to make each training "course" very small and consumable in a short period of time. Have it focus on a specific thing someone would want to do within the domain of the greater problem. This lets people pick and choose which isolated topics (can have pre-req's) they need to take and because they can learn the task quickly they have a high rate of completion.

Creating mountains for people to climb isn't always the best approach. By creating a series of small foothills, learners can pick and choose which tasks they need experience on and I believe this also leads to better skill retention.