I have cycled the bike path from Tai Wai to Tai Po, then to Sheung Shui once on a weekday. The way from Tai Wai and Tai Po was good, I was able to go with decent speed, overtaking without great difficulty. However, the construction progress of the cycle path from Tai Po to Fanling wasn't completed yet (by October).
However, the quality of bike paths in Yuen Long is notoriously bad to the extent that people even avoid it by pushing the bike into the metro for a few stations to "bypass that area".
The following is my feelings:
1. The government encourages cycling as an environmentally-friendly transport mode, *but only outside the urban areas*. It does not encourage anything in the urban except public transit.
2. However, most of the urban areas are suitable for cycling, especially Kowloon. Kowloon is not big, for example, from Mong Kok, only 9 minutes to Nam Cheong Station, 11 minutes to Jordan, 15 minutes to Kowloon City, 20 minutes to Choi Hung and Mei Foo, and in most cases, faster than the bus. Of course, I am not telling you to cycle from Tuen Mun to Tsuen Wan to commute (they are too far away and the bus simply goes on freeways), but it is the best tool to get to areas without the metro and not on the hill (for example Kowloon City), and also good to replace short-haul metro (e.g. 1 to 5 stations) and areas where taking the metro is inconvenient (e.g. Tai Kok Tsui to Mong Kok East) but too far for walking.
3. The reason that the government does not encourage cycling in the area is because it is too busy. That's true. The urban area is really busy. However, in most cases, it is possible to avoid busy roads (e.g. taking Shanghai Street instead of Nathan Road, or Castle Peak Road instead of Cheung Sha Wan Street, where there are a tremendous amount of buses). As the roads are busy, the traffic is not fast and is comparable to my biking speed, so riding in the middle lane is not a problem (the cars cannot go faster than the traffic) Sometimes, it is unavoidable to cycle on a major road to the destination because of design limitation, e.g. Argyle Street, Prince Edward Road East, but it is not a major concern because you always stay on the leftmost lane.
4. I tend to avoid rural areas because the roads are too narrow, with only a single lane each direction, and the lane width is so narrow that two buses have to yield to the other when passing curves.
5. The most problematic thing when biking is taking the bike across the harbour. The Central - TST ferry does not allow bikes, the Wan Chai - TST ferry allows bike, but with a surcharge and many restrictions. Although technically I can disassemble the wheel / fold the bike and push into the metro, it is always *too crowded* to do so (all three cross-harbour metro lines are completely full in peak hours).
I did a round trip between my home in Mong Kok to my home in Tuen Mun in a day, and it took me about 2 hours forward, and about 2 hours and 40 minutes backward. A single trip was about 32 km so it was about 64 km both ways. There were no cycle paths along the whole route and I was able to cycle the whole way on vehicular roads, without getting off my bike so this may be a good suggestion for a trip about 30-50 km.
The following is a route that you can take for reference:
TST Star Ferry -> Kowloon Park Path -> Canton Road -> Ferry Street -> Sham Mong Road -> Po Lun Street -> Mei Lai Road (beware don't get on the bridge to Kwai Chung Road! it lead to a freeway) -> Lai King Hill Road -> Kwai Fuk Road -> Yeung Uk Road -> Hoi Shing Road -> Hoi On Road -> Castle Peak Road (don't get on the bridge near Tai Lam). I would not recommend to go beyond Tuen Mun to Yuen Long because the cycle path is bad. The reverse is similar, but you have to enter Hoi Hing Road -> Tai Chung Road -> Sha Tsui Road to return to Yeung Uk Road in Tsuen Wan, and use Tung Chau Street -> Tai Kok Tsui Road instead of Sham Mong Road because it is not possible to get enter Sham Mong Road from Lai King.