Hi there,
I cycle in from Tranent once or twice a week and the comfort of the cycle really depends on a few factors.
1. Route
Although I live at the south side of Tranent, I tend just to cycle down the A199 to Musselburgh on the main road, then up Milton Road to 'The Range' shop before joining the cycle path (Route 1). I work at Kings' Buildings so I come off before the Innocent Tunnel/Meadows but generally that will be your best/quickest route into central Edinburgh.
In terms of distance and time; it's 10.5miles to KB and that takes me around 40-45mins so you can probably add another 2miles to that if you're going to the West End. Bearing in mind though I use a hybrid bike and am I'm not the fittest! ;)
The elevation of this route is very fair. Downhill to Musselburgh then a short climb up Milton Road. When you join Route 1, that's a very gentle incline to the Innocent tunnel which takes you up to the centre of town. Obviously you have the opposite on the way back. The climb home from Musselburgh to Tranent isn't easy for the newbie but neither is it too tough and the extra effort keeps us healthy.
2. Time
I leave around 6.30-6.45am and the traffic on the road is very light at this time so I have minimal worry in that sense. It is a bit heavier by the time I reach Edinburgh but my on-road route has cycle lanes so I feel as safe as I can be.
On the way home, I leave work around 3-3.30pm and again the traffic is pretty reasonable by and large.
3. Date/Weather
I've put these together as they're related factors.
I'd consider myself more of a fair weather cyclist (and only started cycling to work this year) so try to avoid wet/windy days and the winter due to lack of light. From April to September the weather is fairly good and the light fine. Outwith that you're talking dark mornings and, depending when you leave after work, dark evenings too. Just now is fine but I'm going to have to think about buying lights for my bike if I want to keep cycling in as it will be dark at 6.30am soon enough.
4. Kit
I tend to just wear baggy Endura shorts (with liner) and a base-layer with top. The mornings can be chilly though so a soft-shell is a good idea - especially for the unpredictable weather where the wind and rain can appear at any time.
I have a Deuter Bike 20 rucksack which is ideal for me as it has enough room to fit a change of clothes, my lunch and a laptop. That adds a few kilos but the pack sits up high on my back and it's a quality bit of kit. I genuinely don't notice it's there usually. Well worth £50-60!
5. Other routes
As well as the quickest one above, like others have said there are a few alternatives and you can almost avoid roads altogether if you really prefer it.
Out of Tranent to the South or West can be interesting to join Route 196 and then onto Route 1 at the West of Whitecraig. You can join R196 at various places on the Ormiston Railway Walk (can be wet and muddy on some sections) or at Crossgateshall but that means cycling on a tight road on the approach to a bizarre bridges intersection.
From Whitecraig, Route 1 brings you up past the Monktonhall golf course, Heriot Watt Uni and Newcraighall village before joinging the cycle path at 'The Range' as per my normal route. This whole route is mainly off road but will add up to 5miles to your journey.
I actually like to come home via that route on a Friday and if you do R1, R196 and the Railway Walk to Ormiston before cycling back up the hill to Tranent it's around 15miles in total for me with a few more wee climbs.
You can also cycle down the coast and back. Last week, I finished work early one day and cycled Route 76 right out to Longniddry Railway Walk and up to Haddington and back by the A199. That was closer to 25miles which is plenty enough for me!
All in all, Tranent is a great wee place to cycle from - commute or not. It's only a few miles to the coast road or you can go South into the hills. Gifford, Gullane and North Berwick are great places to cycle to and these can be reached via very quiet roads. You can easily add/remove miles via small changes and Tranent is the ideal base.
Have fun and feel free to ask any questions.