Well done for your progress so far, the hardest part is getting started!
When I was over 16st I took up Spinning which really helped shift the weight when combined with changes to my diet. I found this was good as it is lower impact, maybe some cross training such as cycling and/swimming could help shift some of the weight?
The Garmins with bluetooth in are able to connect to your mobile phone and then use Livetrack to communicate your position to the people you select. If you do not wish to invest in a new Garmin there is an app available called Glympse that provides the same functionality from your mobile phone. Both systems seems to work well and are great for safety, peace of mind and enabling supporters to find you!